Not sure about the editor but here is how to do it in CCP.

It may be the same?

First the image/picture must be the proper size before adding it to the image file area.

Right click over button to be modified. (anywhere actually works)
 
Select Add Image

Drag image into the Grey box.

Highlight the image only and drag out of the box over the desired button.

Right click back on the Grey box and delete.

May or may not work for you.

Good luck