Which version of IL2 do you have? Have you applied any special modifications to this version of IL2?
The main problem is likely to be one of location. If you start with a skin which certainly can be seen by other players (e.g. that is is without doubt in 256 colour format), try to confirm that the folder in which you place this is exactly identical to the folder in which your on-line mates have done.
For example, if you skinned the P-51-D20 aircraft, then the folder would be:
[your drive letter][your path to IL2's folder] \PaintSchemes\Skins\P-51D-20NA
It must be the same as shown above, or the skin you made will not appear in the available selection list of the load-out screen.