I agree with the above advice (i.e. either crop the image, or shrink the image to a size less than or equal to 200x200 pixels), but I have another addition: If you have Photoshop 7, use the "Save for Web" option for your thumbnail rather than the plain "Save" option. For some reason, the former option compresses the file size a lot more effectively than the latter. This is important as the thumbnail has to be less than 15 kilobytes.