Hi Chappa! I can duplicate this- about a year ago, I would get randoms on the png's misbehaving: here's my details as I remember:
to 'cure' the hassles:
1) in the Preview app, ( and maybe other video related= quicktime etc> clear the "recent" listings ( seems the video datas' get " shared"
When saving the file, use ONLY the top view, with (all needed) save point required.
I was also fiddling with (2 . 400 K Jpgs, saved as PNG for imports, 36" high by 60" long
I also remember doing a beta vid about this, but I have recently trashed them from the you tube files, I'll see what I can find.
PS: I also put the imports on a seperate layer, and would keep that closed except when I needed the data theron ( tracing kept seperate of course)