This is a weird one.
I load all my graphics as anti-aliased PNG files, and when I click to change the position of hotspots, with each click it degrades the image (mostly the edges) more and more! at first I didn't realize why my sprites were getting jaggy, but I think I found the culprit.
Note that this only happen if you switch frames using the timeline on the bottom of the picture editor.
Did anyone else notice this?