Animated gif should work too, though I never tried it with GD library. Maybe the gif doesn't work with GD since I heard stories about its malfunction.
Got a pm the other day which was rather interesting. The question was why there was a "403 forbidden" error when he uploaded the .htaccess file. I responded the following by pm, i'll update later if this was the problem.
Quote
Hm, first thing what comes to my mind is that your CHMOD of the directory is not set well. Can you check what the CHMOD of the directory you have put the files in is? CHMOD are file permission of that folder. Most of time you can browse to the folder containing the stuff you want (the signature files) with your favorite ftp program and rightclick it. After you see something like "file permissions", "attributes" or just plain CHMOD option. You can set either codes or click options. I use flashFXP to upload my files and here is how it lookes for me:
http://www.bastijn.nl/zooi/chmod.png => the option CHMOD.
http://www.bastijn.nl/zooi/menu.png => the right click menu when I hit a folder.
Hope this helps, it should be code 750 or so (i use 755). 644 Might not do the trick. Reply what you have and if it is 750 i'll look for other reasons
.