you have to make a sub folder under galleries in the editor. then put all your custom pics in the folder. when in the main editing screen right click on a button and select "move to back". then click on the background, go to the image section on the right, select import. it should automatically open the galleries folder where you will see the sub folder you created, open it, and select the image you want to import. then of course right click on the new back ground and select "move to back", in order to get your button back to the front.

i cant remember the exact size the images need to be, but i find its best to crop them, then resize the pixels, as to keep the images from looking wide or long.

theres probably a better way, but this works for me.