Option: theme_overlay_image

Example: overlay.png
Since: 1.3.8

This option allows you to specify the name of a png file which will be
stretched (if necessary) and displayed on top of the screen. The image
should have transparent areas (alpha channel), so the Amiga display will
show through. You will also normally want to set scaling options (or other
display options) to make the Amiga display output fit the overlay image.