If you're prepared to use pointer environment/high resolution modes, make the graphics into 256 colour (8-bit) GD2 sprites. When printing sprites to the screen (I think Per has an extension to make this easy on his site, though I've forgotten what it's called) the "system" does a degree of mode conversion of the sprites so that 256 colour sprites are printed to mode 32 and 33 screens as well.
Admittedly, some knowledge of PE programming may be needed for this, so probably an idea for the future.
PNG/JPG/BMP can be converted direct to SMSQ/E sprites with the SMSQSpriteConverter (a Windows program) from Marcel Kilgus https://www.kilgus.net/smsqe/sprite-converter/
Note of caution: I've not actually tried this, so can't guarantee if it works or not, this is just me thinking aloud.
EDIT: the sprite write extension is called WSPT on Per's site at https://www.knoware.no/htm/toolkits.htm
More help needed! Mode 32 vs 33
Re: More help needed! Mode 32 vs 33
Just did a quick test of CV2NAT and it works!
Will hopefully have a new version to download in a few days...
Cheers,
Alan

Will hopefully have a new version to download in a few days...
Cheers,
Alan