It's kinda stupid!
Make an avatar sized 100 by 100 pixels in monochrome (that means black and white pixels only) then open it with 'choose file'.
Select the number of frames (this determines how smooth the spin of your avatar will seem). Note that too many frames will make your avatar have a really big file size and it might be too big for JV's file size limit.
Select the camera distance in pixels. (this is the distance of the virtual POV).
Select proportion %, this basically changes the size.
Select thickness in pixels, this is how thick the cookie-cut spinning object will appear to be.
Click submit and then right click on the image produced to save it to your computer, you can then upload it on jv to have a sexy avatar.
OR you can open the GIF in a GIF editing software and add some crazy colour or whatever else.
Also here's the original image if you want to fiddle around with it yourself: