Somehow, accidentally, I tagged an image as an animation, with one parameter having the animation indicator. As far as I know, I didn't do anything to do it, certainly not on purpose, and Animation was turned off as far as I know.
(1) How could I have done it, and how could I avoid it in the future?
(2) How should I undo it? I didn't see any way, so I edited the upr, taking out the code for the animation indicator, and re-saved the upr. Still, when rendering the image, the jpeg had "0100" appended to the name.