theme_overlay_image

The option documentation is properly formatted yet

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.

Source: https://github.com/FrodeSolheim/fs-uae/blob/master/docs/options/theme_overlay_image