A internet forum phrase originally coined in Something Awful forums (like nearly every other internet phrase, really), this is an image with a (hopefully) funny phrase relating to the situation in the thread.
They're usually intended as offensive, but are generally seen as a rather low form of "entertainment" and don't generate much heat in a flame war unless the person or thread it's aimed at is particularly sensitive.
Thread Topic: cute kittens
PostWhore: "My cat is really cute!"
Joe Poster: "My cat is also very cute."
Poster X: image of a man in spandex with the words "THIS THREAD IS GAY" superimposed over it