based on what you typed, you might want to make your visited state read:
background-image: url(gifs/check.gif); <-- just make sure that path is correct!
and add to the other states:
background-image: none;
and close off that last css code with a " } "
that's just looking at your code, i didn't test that. you could try to upload the css file or have this <a href="http://www.htmlhelp.org/tools/csscheck/" target="_blank">css validator</a> check your site.