That's what I figured with the bmp files
Strangely enough x-cart does 'eat' them - should have been some message/barriere implemented that these files can't be used (at least with the generation of thumbnails).
Thanks for the pointer, unfortunately I lack the knowledge to implement this, nobody around having had the same problem and who can help out with a x-cart hack of some sort?
I can live with the fact that bmp files can't be used, but need some solution telling my clients they should upload png or jpg files when they add a bmp.
I now have certain sections in the shop where the original sized 'thumbnails' are shown due to the bmp image (causing huge images breaking up the shops) - must be happening to others as well I figure.