You can but it will be pixelated...crappy. Right click on the picture and save it to your desktop then open it with whatever imaging software is on your computer and magnify it. OR right click on it and go to properties and copy the URL and paste it into the browser...then some browsers have an enlarge feature built in.
"I was thinking the exact same thing about you..."
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.