@Hayley - Unfortunately I don't believe the image "pop test.png" has a transparent background - in fact, it seems to have the gridded canvas with it.
If you are using the images as background inside the button widget, then setting the Fill color for the background as transparent will do the trick. See the comparison when you change the fill from the default to transparent:
Another interesting thing is, I do wonder how you achieve the original setting to put an image inside a button as background but aligned to the bottom, or you used an Image widget to lay on top of the button widget?
Hope it helps,