I am going to say know because of the type of beam and the optics. A par cans light is more spread out making it impossible to project a crisp pattern. An elipsodles light is focused and concetrated making able to project an image. As for an explenation and correct answer you might need to wait for some one else.
Soundman, you are completely right in all ways with your description.
The only possible exception to this would be to use a converging beam PAR or R-Lamp such as a MR-16 type EPN where the beam of light focuses upon a specific point in this case 9mm from the lens, than reverses itself. As with a Leko, this is the point that a pattern is placed as it is one source of light at that point given there is actually two sources of light otherwise on a Leko or Par lamp or anything with a reflector. The first source is the filament, the second is the light reflected off the reflector. Different sources of light equal differnt patterns projected off them thus a really bad projection. Also the larger the filament or point source of light, the larger the dispersion or fuzzyness of the beam projected.
All that said, if you study into the concept of a scene projector or more properly called, a Linnenbach projector, even a "Go Fres" presented in 1991 USITTTheatre Technology Exhibit Section 15, you will find a way to do this with the equipment you have. In the most basic sense, if you take the above description of how a Leko projects a pattern from it's focal center - that place in which all light beams converge in hitting the pattern, than this is fairly easy to reproduce in any single source of light fixture. All you need is that origional source of light without any secondary reflection sources or wide random/soft dispersion lenses. In the case of a Fresnel, it's a fixture without lens or reflector. In the case of a Plano Convex, it's without the reflector and possibly without the lens. In the case of a scoop or PARfixture, it's a done deal as long as there is no reflector being used. Place a clear A-type normal household or better lamp without a reflector into the fixture and it now has a single source of light projecting thru the pattern. You don't need to have a converging beam of light, just one that's parallel and directly coming from a single location. Not it plus bouncing off a reflector.
As a goal to work around, the larger the filament area, the more diffused the beam it will have since light is coming from a greater surface area of light and angles from it projecting onto the pattern will effect how graphic or defined it is. The most graphic source of light to project a pattern is a pin prick mini-source of light. The larger the source, the more angles and fuzzyness. A FLK lamp is probably going to be a better point source than a EHG for patterns. A GLC lamp is going to be better than a FLK for this same filament area idea. A HPL should be about the same as the GLC with a HPR best yet for it's own unique design reasons given the efficiency of the reflector and it's lenses on a fixture also plays a factor. This is on Lekos, for projectors Lekos have nothing on a stage and studio grade arc lamp having down to 4mm dia a light source as opposed to a 6.35x11.68mm filament on a HPR, 9x8mm on a GLC or 13x8.5mm filament on a HPL lamp.
In the case of a PAR 30 fixture, were you to install a normal screw bulb in it instead of the PAR lamp, such as an A-Lamp being a general term for any household bulb in shape, you would have that point source of light projecting on the image, it's just a question of output and filament size. Output because this lamp no longer has reflection to collect up and re-direct the light in a general direction, and filament size in making a useful beam of light projected. The above Linnenbach projectors or Go Fres's in projecting scenery don't need as graphic an image as a Nike logo on the way to their corporate event. Some execuitive would be really pissed with a fuzzy logo. That fuzzy image however is a design style most desired in projected scenery as in a whole back wall of a set that's a projected image. To that you frequently will even add a scrim to further disperse the image.
For a PAR 30 Fixture on a "B" sized pattern, assuming you want a fairly graphic image, we have to take as a given a maximum A-Lamp bulb length of say 4.11/16" for a PAR 30 long - given it would fit into a PAR 30 can. Otherwise our MOL as it's called for maximum overall length is 3.5/8" which is fairly short to try and match up a high wattage A-Lamp to.
This is important the more wattage when balanced with compact filament, the better the image will be. This is all taking it for a given that the bulb has to physically fit into the fixture both in dia. and length.
If your fixture can take a long PAR 30 instead of the normal short ones, than it's possible to go up to a 500w T-4 lamp in wattage given the length of the screw based lamp will fit into the fixture. Such lamps as the GE owned EYE brand JD500w/E2 or Bulbrite Q500CL/E26 would work well for this given a cc-8 filament is fairly small in are instead of being linear as if a RSC double ended type lamp which is also possible.
You could reasonably expect that the intensity of the image on the wall would be similar to that of a 200w lamp at the same distance. Should be decently bright above normal lighting, but not a huge amount of punch to it. Short of changing your lamp base and adding a higher wattage lamp with a possibly larger filament it's hard to beat this in output. One key factor to figure on is not only filament size and type but it's orientation. You could mount a very good BTR lamp into the fixture given a change of lamp base, but since it's filament is perpendicular to the axis of the lamp, this in a PAR type fixture would not be very bright. With the above 500w T-4 lamps, it probably is very similar to a double ended lamp with a long filament stretched between two supports instead of a compact filament as per on a Leko lamp.
The next best source given the above 500w lamps are long filaments, or at least one of interest would be a 400G/FL lamp such as you would find used in a 10" scoop. Cheap lamp to get, bright, but it's filament is not very compact meaning your image would be fuzzy. Next best lamps that might fit are 250w even 150 and 100 watt more normal lamps. Given they are not too long and that's a specification thing, they should also work if clear not frosted. A frosted lamp becomes a huge filament because the light projects out in all directions from the frosted surface. It's as if there is one huge filament. Not good for a patternprojector.
Store bought lamps would probably limit you to about a 60w lamp in clear, perhaps 100w. This will project an image, but only figure on about 1/3 of it's total light coming out of the front - the rest is wasted. Reflection is efficiency, remove that reflection and the light just turns into heat. Expect that your PAR 30 fixture might suffer from heat problems.
Give it a shot, might work well enough, but chances are you are going to need either some type of Leko or to modify a fixture that has the axis of the lamp perpendicular to the opening it's shining out of.
In a lobby, I would look towards a more architectural based G-12 type metal hallide HMILeko. Such fixtures project images well and have very long lamp lives. What brand between Altman, ETC or others is up to you and the price, as with going mini-Leko or normal Leko in size. For perminant install as long as you optimize what projection size you need for the fixture it's going to work. There is also some other types of projectors that might be more reasonable in price. If it's a question of a perminant projection, you will be most cost effective in going with something that's designed even engineered to project things than something you come up with.
The G-12 based lamps based around the Philips CDM lamp are available in two main color temperatures that are directly relatable to that of a stageLeko, only the hours of life are much better thus more cost effective both in wattage at 150w, and lamp life. A type 830 lamp will be a little more amber and dim than a S-4 with a HPL 575w/C lamp, and the 942 style will be more bright than that same fixture and lamp.
Go with a CDM-SA/T150/942 lamp, it's for the most part the most cost effective bright lamp. GE actually makes the brightest lamp of the type, but Philips in being most available will probably be cheaper. Otherwise the most high output/color temperature lamp available is the Osram HSD 150/70 lamp. We are talking better than follow spot in color temperature but more limited life.
I know American Dj makes 'crap' but they do have a 'gobo spot' on the market. It accepts an EXN lamp and the unit has a transformer built in. It does accept an odd-shaped pattern (about 1.5", can be custom made) but the output is not too hot, which would probably be good for a lobby. I believe they cost around $50.00. Or you could check out www.cheaplights.com. I'm not for certain, but I think they may have some affordable gobo projectors.