Strange reflection in exported SWF file.

I made a test animation with fire. See
I downloaded a Youtube video with fire.
Converted it with Free Video to JPeg converter into frames.
Changed the black background color into transparent with Gimp.
Imported these frames in Pencil. Exported it as Flash/SWF.

The problem: A strange reflection appears at the top of the animation.
Normally I draw in Inkscape and do not have this problem.
Neither any of the imported frames show this effect at the top.

I use Pencil 0.4.4b because it can export as Flash (and my version Pencil2D does not).

Let me know if you know!
Thanks a lot,