Check it out: http://travellermap.com/test/ImageTest.html
Call for input: what other URLs would be good to have in a corpus of tests as reference images?
While building this I compared my development machine vs. the live site, and was struck once again by how badly the text rendering is done on the server since it stands out in the image diffs. I was able to make a one line change which dramatically improves the text and glyph quality, so if you've saved off any bitmaps recently you probably want to redo them. It's embarrassing that I didn't do this earlier!
I also enabled CORS headers for image generating APIs, but that's probably not of use to anyone unless you're using WebGL to render map tiles or some such.