Agreeing with IronJelly. No avatars and no pictures in signature. That's how I like it. And if there will be avatars very limited size and no freaking flash pics. I hate when people add some long flash videos in their avatar/signature.
Avatars aren't that bad when small enough and not animated... but afaik php doesn't really have a decent way of forcing that, unless forcing to upload them to forum server (instead of uploading for example to imageshack and linking from there) and banning .gifs (and all other animatable extensions). Not worth it I'd say.
A definite no to signatures. Even only text ones - why would anyone want to read some "cool" phrases like for example "my granny ROCKS" some few hundred times?