Hi! Kielen inspired me to fiddle around with the Gimp, and as I am not very patient ~ LOL ~ I managed to stumble on the way to make an Animated Avatar using the Gimp!!
Step 1 - Open up the Gimp and make a new picture, size 100 by 100 (or whatever size your website specifies, as each one has different sizes allowed)

(also, click on Dialogs / Create New Dock / Layers, Channels & Paths - so that you can see your layers)
Minimize this New Pic
Step 2 - Import your desired pictures! (most of you know how to do this next part, for those who don't, please follow along)
Go to File / Open / choose whatever pic you want
Once it's open, go to Tools / Transform Tools / Scale ~ then scale your pic to the wanted size.

Then go to Edit / Cut and cut pic, then close image without saving
Then go back to your New Pic and choose Edit / Paste then go to Layer / New Layer and there you go, you've created a layer!
Step 3 - Repeat Step 2 for each added layer
Step 4 - *optional* you can add any flashy extras to each layer at this stage from you filters bar.
Step 5 - Now you are ready to make the Avatar!
Go to File / Save & under 'Select File Type" choose Gif Image ~ make sure you give your avatar a cool name and press save
This box will pop up ~ click on the highlighted buttons ~ press Export

Then this box will pop up ~ leave it marked the way I have it, except for the "Delay between frames where unspecified" Just remember, the higher the number, the slower your frames will change. Press Save
Step 6 - Enjoy your new Avatar!!

I hope this helped, and that I didn't confuse you!!