ya some people's avatars are small(like mine) and will look pixilated if you try to make them bigger, but other people's are big (like Steve_HKmtrsprts ) and you can right click, select properties and paste the url into your browser like Kritter said. Most of those pics probally come from larger ones, and im sure if you pm people and ask they will point you to where you can find a bigger version of the same pic.
97 Ranger 4x4
02 CR125