Using taxonomy term images seems to be a good way to use icons instead of text-based tags, especially for a vocabulary with few terms that can be clearly visually distinguished.
However, some icons simply wouldn't show up - with no reproducable pattern. Turns out there is a patch that unfortunately hasn't been included so far. Using that means that taxonomy term images in a view can now be restricted to a vocabulary. Unfortunately - since somebody else installed the patch - I'm not completely sure any more but I think it was this one.)
It is a bit disconcerting that the term image doesn't show up with the taxonomy term itself, so at times the best way of finding out which icons I had used was to go to a node that should have it. There actually is supposed to be a list of all taxonomy terms, but at least for me that has a bug, and claims that there is no image for any of the terms to whom I've attached them.
Also good to remember: If you replace a term image, use a different file name, otherwise apparently the old file still gets used.