First of all, greetings, and welcome to the forum! I hope that your time here proves to be positive! 
As to your question… I’d guess that what you’re seeing is the default button-geometry showing up behind your image.
How are you applying your texture? Are you using the “frameTexture” parameter, or the “image” parameter?
If you’re using “frameTexture”, then I’d suggest setting the “relief” parameter to “DGG.FLAT”.
Conversely, if you’re using “image”, then I’m not certain–I usually use “frameTexture”–but I think that setting the “relief” to “None” should do the trick.
(Either way along with calling “setTransparency(True)” on the button, of course.)