what kind of image ? Editing css , you can get a background and use crazy fonts. Is it not enough ?
IMHO, your request reveals an issue of substance : don't give anything else than a good content, as true as possible. Your work is to distinguish scalable and permanent knowledges to maintain a nice site, nice by its contents. Or open a blog and connect it to the facebook and other likes :)
ps : for the facebook default image n just add the meta
<meta property='og:image' content='IMAGE URL' />
in the header using the admin page /admin/layout , where IMAGE URL is the image URL.