this is really no big deal, but I think the documentation should contain a warning that downscaled PNG images will look really ugly if they contain an alpha channel:
I spent over an hour trying to track the source of the problem. This (unsurprisingly) occurs both in image buttons (where I had the problem) and in Image objects when used directly and is obviously because alpha-testing is used instead of alpha blending. I completely understand why it’s done, but it is a bit counterintuitive.
edit: What makes this even more surprising is that with
image.autoScale = true there is perfect alpha blending, just not when stretching. So alpha testing vs blending might not even be the root cause of this.