Sheer frustration is using a basically okay program with a buggy GUI. LView is that program for me today -- it's been around for about ten years (starting out as a simple bitmap editor, but now woozy with vectors and layers and whatnot), and keeps sprouting new features. But it's buggy in horrible ways that are totally impossible to demonstrate without actually videotaping the screen, because they're all about select this, click there and dry tragging to there, choose "Paste" from that menu, etc. It's like trying to write a prose description of how to make a shoelace-tying knot, except worse.
Maybe I should have known it was a cursed program when I saw that it doesn't support PNG, not even just for save-as.