Here's what you are looking at. First off, you are looking to create the content that will be presented. I am assuming that your picture frame will be in landscape (normal) orientation and not portrait (turned 90 degrees to be taller than wide). When you are creating the content, you will generally want to fill the entire projected surface, thus sticking with the wide aspect images. This of course is part of the design process, how you ultimately want it to look. Do you want there to be blank parts of your
screen if you have SD content? Do you want to strech or
zoom your content so that it fills the
screen? Do you want to layer your
image so that the SD content is on top of a background which fills the rest of the
screen instead of video black? Some of these decisions may be made by what your content is. If you have a widescreen TV that you are able to change the
aspect ratio between SD and HD, then
play some of your content on there and
switch between the two to see how it looks.
As has been mentioned, it is often desirable to understand how you intend to display your content before creating it. You have one step done by knowing that you will be projecting HD onto a 16:9 surface. Do you know what the resolution of your
projector is? It will be best if you
send the native resolution to your
projector. This way, the
image will look more as you intended it to look as opposed to the
projector needing to process the signal into something it can project. Do you know what your playback device will be? Since you are looking at
Mac editing software, then you will likely want to look at
Qlab. However, it will be best if you have this program on a machine dedicated to show use. This means that you will need to strip down unnecessary applications which can potentially slow it down. The nice thing about
Qlab is that you can rent it for use during your presentation and not need to worry about it setting you back too much. You can also look at a free program called Video Projection Tools. It has its quirks, but I have used it in a pinch. It is made to run on either a
Mac or PC. Of course, if you don't need any processing, you could
play it back on something as simple as a DVD player (not recommended).
You will also need to consider how to get the signal from the source (your computer) to your
projector as well as how you intend to control it. Will you have an operator or have it triggered by some other source, such as a lighting
console?