Yeah that's what i had in mind, just like mortem.txt. If the option is turned on, backup graveyard.png in the mortem folder. I suppose this and mortem.txt backup should be kept as two different options, that way one can keep only the text (or only the png) files if he/she wants. Also, i said screenshot, but i really meant just the tiles and sprites, not the minimap and HUD, if this is not too much trouble.
This leads me to another suggestion, to avoid having tons and tons of mortem files, would it be possible to set a maximum number of files in the mortem folder, with the oldest files getting deleted as new ones are created? I don't know if people would use this or if it is useful at all, i only use manual backups after a memorable session.