AFAIK, white is handled as transparent in IL2. And remember to save it as indexed .bmp, just like the skins.
Ow and the file should be 256x512, the top 256x256 part is where the left side's noseart goes, bottom part is where the right side's go. No need to mirror the text, but if you want the picture to point to the same direction on both side, it needs to be mirrored.
Here's the one I made, the top part showed in-game as the pic with grey background, the lower part showed up with transparent background, so white should be the background color.
