I would personally prefer 100x100 or 50x50 avatars to 48x48 or 96x96 (mainly because those are the sizes used on other sites - this is the only site I go to which uses 48x48). Though I don't actually see a need to enlarge the avatars here - they're a good size for the forum layout, and seem to work well (for reference, I use JCF blue).

As far as images in signatures go, they're all very well but some of us still use dialup. Just because you have a 10mb/s LAN connection to the internet doesn't mean we all do. I know people who still use 48kb/s dialup - while at university, I had to put up with dialup.