Facebook - The optimal size for posting an image to a page wall
What I think is the optimal size for Facebook page wall images
Facebook is pretty good at being smart about posting images to a page wall, it tries to scale them properly but in order to make sure your image gets posted properly you should probably stick to 400 x 400 pixels.
If you put the effort in up front to optimise your image for a Facebook wall you can maintain all the control and can be sure it will look decent when people find it.
If you don't like the idea of square images I would suggest that the height is probably more flexible than the width.