I am sure that there is a way to turn picture rendering from external URL's off.
If it is not a simple machines setting, then I am sure there is a way to do it with web server (apache).
If anyone needs help investigating, I would be happy to lend a hand.
I also noticed that there is no CAPTCHA (squiggly letter image) to sign up for the site.
en.wikipedia.org/wiki/Captcha That means that spam bots can write scripts to automate the login creation and posting process very easily.
I am sure simple machines has IP logging capabilities so there should be a way to filter for IP/IP ranges (not a long term solution, but if it is one spammer, we may be able to stop it).
GUE probably has people more knowledgeable than myself in these area's, but this would be my gut suggestion.