It's been a long time since I have done this. There is a folder of BMP images in the console, you can import new ones as well. The fixtureprofile references an image in that folder. There may be a way to change it in the GUI, but I know you can exit the fixtureprofile in any text editor(.XML) and change what file is associated with the gobo. You will probably have better luck on http://ma-share.net/