Hi, welcome,
If you want to contribute artwork and such to this project, you would want to get in contact with forum member Winter, who is in charge of approving that type of stuff.
Second, when you say "mod" I'm not sure what you're talking about - if you're referring to this project, UFO: Alien Invasion is not a "mod" of anything - It is an entirely independent game, simply using a modified version of the quake 2 graphics engine (but not based upon the quake games) and inspired by the old XCOM/UFO games. (Also used is SDL and other libraries, one of the coders could explain it much better...)
We welcome anyone who wants to contribute and further develop this project, although since you're new I'd suggest glancing at the wiki to get familiar with it.
Edit: Also listed in the wiki are TODO lists that detail much of where we are before the next release and what we need done to get there.
Given that you've had quite a bit of experience doing skins, as you said, we could certainly use your skills here.