When you work in Photoshop or Any other program then you work in 24 bit mode.
IL-2 expects its skins in 8 bit mode.
You can easily see by the filesize I guess your file will be 3 MB big. Note that all working skins are 1 MB.
You have to convert the skin to 8 bit/ 256 colors. Often called Indexed color.
You can do that with the program you use. But often Bright gives a better result.
You can find Bright with a batch file that turns your current 3 MB big BMP into a proper BMP for IL-2 or you can find it as a plugin for Photoshop. Then you can use File-Export-Bright from Photoshop to have your skins saved in the proper format.