Ok I disagree with some people here.
I honestly feel that most people are using a screen resolution of 800x600, so why give them a super huge picture? It's not fun to scroll while you jerk.
My pictures are cropped to where the longest side is 700pixels. But they are cropped so the model is taking up the entire space of the picture. Less background makes for bigger looking pictures, but not actually too big that they have to scroll much to view it.
The file sizes range from 80k-175k depending on the picture.