I've tried with another picture, one from the official PNG page :

http://www.libpng.org/pub/png/img_png/OwlAlpha-0.5.png

In this size, there is the same problem, if I reduce it to 320x240 with The Gimp and launch the program again, it is perfect.

So, yes, it's really related to the image size.

Does someone see a way to have a perfect move ? In this simple case, my widget is a QLabel with setPixmap and setMask and I don't override paint event, but I think I have another code snippet I've written with a paint event, double buffering... but it seems to be problematic as well.