Sometime images might need tweaking in order to properly appear in PDF. That was my experience but with different library.
I had few cases where all parameters were right, no error message but no image shown either. Then after playing a bit with formats/scale
it would pop up 'all of a sudden'. Might as well be something trivial like that, that is why I asked.

When I tried the same thing interactively in Acrobat, the image showed up on the button with no problem. But I can try the same code and scaling the image first.