Automatically adding an icon to each post?

Hi all, is there an easy way to have a category-based icon appear in each post? I have a different visual cue for each category … want it to appear in the top right of every post with that tag.

So, I guess there are two parts to this – (a) what would the code be to dynamically pull the correct image based on which category the post is in, and (b) how to deal with the fact that most posts have multiple category tags, and I just want it to use the image for one of them? (say, the first one – I wish there were a way to make each post have a “primary” tag in WP, but I don’t believe there is)