I'd check the agreement that you agreed to. From what I remember, most allow you to use the Clipart for personal use, and so on a non-commercial site then you should be ok.
Not sure about threads, but I would be tempted to say that they are not owned by anyone, so this shouldn't be a problem.
There shouldn't be a problem posting a link in the forum, though I would suggest using the Web Design forum and making it clear that its your site.
Try searching the forums for a list of sites that help you with web design.
Mainly my opinions on the matter, I can't be sure that I'm correct.