I had trouble using my own photos on the User Backdrops panel of iChat's Video Effects feature. I would drag the photo to any of the blank areas, the photo would appear, but when I selected it for the video background, I only received a blank gray background. I talked to two different Apple tech support people and found out it was because the photos were taken on a Sony camera. The solution was to do any edit to the image in iPhoto (or other image editor) and save, then drag the photo to iChat. When I did that, all was well.
I also couldn't figure out how to remove my images from the User Backdrops panel. It turns out that all you need to do is drag another image on top off the one you don't want, and the original will be replaced with the new one.
[robg adds: I couldn't figure out any way to actually remove the images from the User Backgrounds panel short of editing (the scalpel) or trashing (the hammer) iChat's preferences file. If you choose the scalpel, quit iChat then open com.apple.iChat.plist in Property List Editor (or PListEdit Pro or a pure text editor if you like raw XML). Look for UserBackdropsPrefs, and delete the entire entry (or open it and delete just the reference to one image, if you're just trying to remove one). Save the file, relaunch iChat, and all is back to normal.]
Mac OS X Hints
http://hints.macworld.com/article.php?story=20080229114129143