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!!