The title says it all but am willing to clairify if you need more clarification.
Thanks in advance!
You mena appearance or click area?
Click area
Im not too sure about creating a custom click area for a button but you can add images to buttons by adding a button into your UMG canvas and going into the appearance → Style menu and in there you can then set an image for the button for a few different circumstances such as the normal look of the button, how it should look when you hover over it and what it should look like when pressed.
Problem is though, I wanted the buttons quite near to each other and simply changing the appearance would create overlapping buttons.