@dc42 Thanks for the help. What I have found, so far, is that Prusaslicer does not export QOI thumbnails yet, even if the microcode style is set as RepRapFirmware (I think they are PNG).
I have a small program that adds thumbnails into my Simplify3d gcode; unfortunately they are not QOI files (JPEG makes sense), so at the moment I can't generate QOI thumbnails...