What you need :
- create the
overlays
folder + sub directory with the same name of the system - add there your .png, a romname.cfg + romname.zip.cfg
So here is the basic info through an example
- System : mame
- Rom : double dragon (ddragon.zip)
Folder : /recalbox/share/overlays/mame
Files :
- /recalbox/share/overlays/mame/ddragon.zip.cfg :
input_overlay = /recalbox/share/overlays/mame/ddragon.cfg
- /recalbox/share/overlays/mame/ddragon.cfg :
overlays = 1
overlay0_overlay = ddragon.png
overlay0_full_screen = true
overlay0_descs = 0
So see the link : recalbox will look for overlays/<system>/<rom.extension>.cfg which itself refers to another .cfg (the file doesn't need to be named like the rom, up to you to customize the input_overlay parameter). Same goes for the overlay name
these are the very minimum requirements. You can of course add some custom retroarch parameters to ddragon.zip.cfg
Most of times you'd force the viewports