First, why does there need to be an image for hot topics? I've seen this on other forums, and don't really see the point to it. It baits people to certain discussions while leaving others in the dust.
Secondly, I think that the greyed out text and post-icon is much clearer than an image. And the same with the colored boxes that show that there has been a new post. Colored = new post. No color = no new. How much clearer do you need it?
Thirdly, images aren't necessarily good. In fact, most people that I know that have changed from other systems to this one appreciate the fact that there are really no images in the entire thing.
However, if you want to add them, you can. Just look around the PunRes wiki.