I am having the same problem.
I have a transparent .png image that I want to use as. background. As you can see, the theme editor has a black overlay and black background but the transparent image has a green background. I've removed that colour from every conceivable setting in that theme and still it persists.
Changing the theme doesn't change that green background on the transparent image.
I produced a work around image but I would love to know how to solve this issue.