You should resize the photos really before uploading to forums. Not just because you may exceed any size limits but huge pictures cause the page to load slowly for people on slower broadband connections and on some mobiles and even PC's the pictures are too big to actually view properly. I find a size of 700-800 pixel wide is perfect for most forums.
There are a few members who have posted huge pictures in their gallery and I can only see the left half of the pictures on my phone. :-(
An avatar is the little picture that is next to your login name on the forum. My avatar is the little pic of my limelight green corsa (well an artists impression)