There's nothing tricky about changing the image for a button(I did it lots of times), as long as you follow a couple of simple steps:
- your button image must have a transparent background
- your button must be set as Custom (and NOT Round Rect)
- your button's background color must have Opacity=0
In order to change the button background color, when you change the color make sure you click on the left side of the color combobox, so that you get presented the color picker. There, you'll have to change the Opacity to 0. 
See the image below for a simple demo.

Good luck with your button