There is a requirement that images are at least 1024 pixels in width or height. Why? A few reasons. The first is that this is a site about photography. It is too easy to make an average photo look good at a lower resolution.
The second is that this is site about birds, and for the birds (no pun intended). A photo that is too small to be printed or to be used as a screen saver is less likely to make it out into the world and have a positive impact. Ultimately, this site has a message about conservation, and tries to acheive it by showing the world the beauty of birds in their natural environment.
Lastly, today's monitors are getting bigger and bigger. 1024x1024 only takes a small part of a new 27" monitor, which cost under $500.