Facebook picture quality on macbook differs...?

hi, i have a 13" macbook pro and my friend has a 15" macbook pro. i was on her laptop and noticed a surprising difference in picture quality on facebook. on my laptop, the quality of photos are poor and on hers, the quality of the same photos are perfect! does anyone know why? my screen resolution is set to 1280x800, which is the best option, so that's not the problem. take note that i'm talking about pictures that have already been uploaded to the web, not image files saved on the computers. thanks for any advice!