I'm currently trying out http://www.random-image.net/ as a backup just in case SignAvatar doesn't come back. It seems a little slow, but you can do both Avatars and Sigs and then some. You create a name for the link and upload whatever images you want into that "folder" and it will randomize through them. So technically you can have multiple sites sigs and avatars if you please. Just again, seems a bit slow at times.
Although.. the link for my avatar isn't working it seems. Poo.
Oh.. and it has a 1500 view limit in 24 hours. Bleh. Maybe not so great now.
Edit:
I just broke that grumpybumpers site it seems. I made an account and tried to upload my avatars and it's now dead. lol
So.. how do I get the grumpybumpers link to work for my avatar? It's a .gif so it's rejecting it.