a note about the avatar size
Don't worry too terribly much about the size of the image you want to use, as the forum should resize it for you when you upload it.
On a sort-of related note, long user IDs like mine aren't respecting the border lines.
Test to see if it worked...
EDIT: WOOT!
Can you change em? Not sure about this one, and when I go to upload another one it just reverts back even if I delete it first...
Can you try it again now?
If that still doesn't work, I'll poke around further and see what else can be done.
This was an issue for someone else already, too.
Edited to add: I tried it out on a test account, and I think this is what you have to do... go in and delete the current picture and hit save. Then come back and add a new picture and hit save once again. Once I did that, my profile page still showed the old picture until I refreshed in my browser.
So maybe the key is to make sure to hit save in between each step?
Actually, looking at this on my IPhone, it may of just been MU computers cache as I now see the icon using a Jazz drawing I did 
so looks like I have the right one now. Thanks Bobbi!