While on the face of it your question might seem simple, providing an accurate answer is not.
The first problem is the definition of “my logo”, which presages a discussion about what makes the logo “yours”, and what rights accrue to that ownership. The answer depends to a significant degree on where you live or work (as copyright laws vary somewhat from country to country), though there are international laws (Berne Convention) that give you some rights in almost any case if you actually “own” the logo.
Similar, though somewhat different, questions arise about the “ownership” of your “forum name”.
There are law firms that charge a lot of money for answering such questions, and make great amounts of money aruging both sides of these types of disputes. They also have liability insurance to provide redress if they should give “wrong” advice or “incorrect” information. That said, you should recognize that any information I provide, and probably that provided by any other anonymous poster to an internet forum, though it may very well be correct, should be viewed with healthy skepticism, and is probably worth exactly what you paid for it (which is nothing)
Having qualified what follows with the preceding disclaimer, if you hold copyright on the logo, or have established your forum name as a trademark or service mark, you can request that Dreamhost remove the content from their servers by submitting a DMCA “take down” notice. As a practical matter, I suggest you first contact those responsible for the site where your logo is being used and initiate a discussion about your concerns. Often, simply educating them about the issue and informing them of the problem can produce the desired results.
There are hundreds of other links on the web (just use google) about this subject, and Wikipedia has a very good collection of articles on copyright, as do various government websites ( example U.S. government site devoted to copyright).Good Luck!