The ImageUrl field of the PackageForm provides a URL where an image for the extension can be found. The image will be used in the configure "Find Extensions" interface, and should be for the extension, and not for the company/project that provided the extension (though you may choose to incorporate corporate logos into the image, should your conscience and your corporate policies require it).

Images should be pre-scaled so that they aren't too big when displayed from within configure, and should be as square as possible, as they may be shrunk and forced to be square when they are thumbnailed.