Whether or not you should allow automatic signatures and, if so, in which form, is of course a matter of great debate. Give users the freedom, and they shall abuse such a power.
I personally find automatic signatures, such as they appear on typical phpBB forums, incredible annoying, because not only do they allow for extremely flashy images, which contrast enormously with the website's colour scheme (which makes the whole page harder to read), but also animated GIFs, which can be even more annoying.
I'm not saying you shouldn't allow animated GIFs on a forum, but not in a user's automatic signature.
Perhaps the best solution would be to allow only a small amount of text, with minimal mark-up, and display the text under each post in a small font and a light color.