Before someone says 'yes' to something like this, they probably want to know a few things:
1) Are you serious? That is, are you going to finish, polish, and maintain this mod, or would they be throwing away their time on something that will be slapped together?
2) How much work are we talking about? A frigate is easy. A destroyer, a little harder. Cruisers and up I'd insist on payment or some kind of code-for-art trade. Not to mention how many revisions you might want, because there's a decent chunk of back-and-forth involved.
It's a good idea to pick an existing free sprite, make a ship, and let people see that you finish and polish things before making requests.