I checked the MFD textures in my Blackhawk, and they actually use the Comanche textures as it was copied from that cockpit. The texture name is COMANCHE_MFDx.tga, where x is a number depending on which MFD it's used on. So I think the Comanche also uses tga files. The texture itself is actually 1x1 pixel dark grey square when you open it in photoshop, and it's then just tiled across the surface.
@Banita, do the BMP textures need to be used in the older style cockpits for them to work?
I also checked the extracted textures from the textures in the game, and they're all TGA files. I'm not sure if the converter did that though.
It's pretty confusing now, which textures work and which don't.