A lot of forums have ways of dealing with spammers, but many of them cause some inconvenience for actual users. This is one of the worse ones I've seen in terms of spammers showing up, and I don't think it's because the mods aren't doing their job.
Maybe disallowing images and/or links with the [ img ] tag until users have a certain number of posts would help. Legitimate new users could still attach images, but the spammer accounts could be turned off before they spam their spam.
There's a browser extension called AdBlock Plus available for most browsers. It's intended to block ads, but if you don't mind ads that support all the free websites you visit, there's a Malware Protection list that only blocks malware. You can select only that list and you'll never notice anything different. That might offer some protection, but I doubt it's as good as antivirus software.
At work they block anything that's hosted outside the US. It's a bit extreme, but it does seem to help with stuff like this since it's harder to find a US hosting provider that will put up with spammers, versus elsewhere in the world.
Note that even though these spammers might be posting invisible images, they can set the server to send you whatever they want. They could embed a virus (not sure if there's a way to do that), or later on they could switch it so the images are visible spam.