On custom Facebook pages images must exist on your own server (or somewhere else on the internet) and linked to from the Facebook page code.
The only images that can be uploaded directly to Facebook are the logo for your fan page and the image used on your profile page. For the best logos and other designs, people need to check Graphically and get the best graphic designs.
FBML provides code that will pull your profile or logo picture to your fan page so you don’t have to link to it from a different server.
Here’s how you do it.