Nose Art format for CloD?

[VF-31 Killer 'B']

In my plane selection screen there's a box for 'nose art left' and another for 'nose art right'.
I added a 'noseart' folder next to the 'skins' folder under 'paints' and now the files I put in there show up in the selection boxes but not ON my aircraft.
I sized the pictures to 2048 x 2048 and tried .bmp and .png.
The Luftwaffe markings eg. Griffons, Green Hearts etc. are .mrk...what the heck is that? I have no save as option for .mrk