Dear list, I'm sorry to have to complicated the matter yet further.
To avoid sending attachments to the list I posted two more files to the bug tracker on the same topic/bug ( https://savannah.nongnu.org/bugs/index.php?32180 ), as promised. Both are 750x160 .png files (like the one already send). One glitches, the other doesn't. I don't know enough about the anatomy of .png files to say anything sensible about this but I do know that this means there is some non-trivial weirdness here or at the very least that dimensions in pixels aren't the only thing that affects whether or not a given texture displays correctly. I'm on a NVidea 6200 AGPx8 (not exactly new but the fastest that this motherboard will take) with binary-blob drivers but we saw the same sort of unusual behaviour in software rendering on Linux so I'm not convinced it'd be the card that's to blame. Yours, Kas.
