I think it highly unlikely that you'll be able to upload custom decals, as in designed by your own fair hand, for a number of reasons :-
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- It would take ~2 picoseconds before people were flying around with various crudely drawn genetalia on their ships.
- Whatever was uploaded would have to be downloaded to every single client who saw your ship. This is very resource and bandwidth intensive. There's a reason all of the graphics are installed on your PC as part of the game install and not downloaded on the fly
- Did I mention the genetalia thing?
Why draw something crudely when there is so much free pr0n on teh interwebz?