Yes, the GIF works perfectly for me. The movie maker files do not load for me. If the GIF file is on your computer, Xilo, I don't see why it wouldn't work. It is 100x100 pixels. an Avi is 80x80. The UBB software should be able to resize it beacuse it is close in size. I make my avi's 100x100 and upload them and they get resized and show up in the forum.
The only thing I can think is you are putting a web address for the location of your avi. I would try it again, hit the Browse button and click on the GIF file from there.