Unless the images in question are housed at FR, there is no extra load on their server. 95+% of all image-laden posts/threads are loading those images from elsewhere.
The problem FR has been having is tied (from what I can see) to inbound traffic jamming the server (potentially from a DOS or Ping attack - or it could be something more innocuous, even), or to overall traffic on the backbone routers that serve FR.
It's definitely not image-based. Actually, FR does one of the better jobs of image-handling that I've seen (and I've worked on several of the busier sites on the web).