I'm opening an animation and replacing the current pngs in each frame with new pngs which are exactly the same size. after saving the animation, the frames lose their vertical alignment and the pngs, which were in an orderly grid, overlap each other vertically. driving me crazy.. any guidance appreciated.
These problems are nearly always caused by dodgy collision polygons and uneven Origin points in the animation.
You can amend both in the Image Editor - see Image Points dialog.