Figured I would throw some ideas into the hat here for the decal tool's long term plans just in the case some of my ideas have not been thought of yet, though I highly doubt that.
If vetting player decals becomes an issue I wouldnt be against a system in which each decal imported has a small fee, but is available to all members of your faction/company. another MMO I played has an import for clan and alliance images to allow display of them on a shoulder patch, and the fee is ~$5(thats their fee, not what I am suggesting) in proccessing fees, but that system is for the entire clan or alliance respectively as its for their main logo. Having a system not too disimilar may be more monetarily long term viable for Frozenbyte, while allowing players more freedom. Alternatively the vetting could be croudsourced by a council of the playerbase, and left to its own devices as long as the vetting is done to Frozenbytes specs. If something makes it through that shouldnt have remove the users who voted the wrong way from the council automatically by having it all done through the starbase website.
Another option for long term viability is something like the steam community workshop (since this is going to be primarily if not exclusively a steam game from what I can tell) voting on things the players want added to the game (steam already has this system in place for developers), and then when they are added, just drop them in the prebuilt library of decals for all, minimizing repeated requests for the same or similar decals, while maximizing the time used to vet the images by only needing to check the things the community voted for.
I think colour shifting of different zones of a decal would be very useful for everyone, so you could for example, have a pinstripe with three colour channels, and the player when adding the decal to their ship gets to choose each colour channel and that gets saved on a per ship metadata basis. Having this on the library of default in game decals could add a significant ammount of reusability too, lessening the desire/need for player made decals too, and as such lessening the load on the vetting process for player implemented decals.