Everyone calm down!
This file (C:\Users\Tom\Documents\1C SoftClub\il-2 sturmovik cliffs of dover\cache\picture.bmp) is a temporary file! NO MATTER how big the skin.jpg!
I deleted all files from cache folder, before i run the game tonight (I tested only offline). When i load a custom skin in the game, the 12MB picture.bmp immediately appears (my own skin is 150k .jpg). And immediately appears a custom numbered
.dds texture-file too. So... it"s just a temporary uncompressed picture, which the game create and uses to convert the skins.