I think a small signature for the purpose of advertising one's forum is okay.
It is those over-sized signatures with the images that should not be allowed to take up a lot of space.
Signatures seem to be accepted by a lot of folks.
Couldn't have said it better myself, @womenonly.
A small link or image in a siggie does not get in anybody's way. Then it is simple for anybody to click on it if they see fit. So the website is being advertised (for free) every time they post here. Like you, I don't like large images in siggies, because they detract too much attention from the subject in hand.