i understand what you are saying but you can't copyright drawing a grenade or a pistol or a rifle, you can copyright your drawing of a grenade, pistol or rifle, so anyone can draw their own, when copyright comes into place is when you are using the original art work or something that will mislead people into believing that your artwork is the original artwork. And don't worry i know exactly that most of the time logic does not apply IP law 
for example Xenonauts, is a derivative work, but no one in their right mind will say that they are violating any copyright, implementation is key
If some of the new graphics are based on the original graphics, either by sharing certain pixels, being recolours, or even something more subtle, then arguably laws about derivitive works apply. I doubt the owners of the XCOM IP would care and would probably even consider such derivative works positive, especially in a project that requires you buy their game to install it.
Granted anything that is obviously not based on XCOM IP should be okay to use, but for instance my soldier customisation images that I'm working on? They're not necessarily okay in every country. (They'd probably be ruled fair use in some, but that's not really the point.
However any derivative work may not be compatible with OpenXcom's license and may be vulnerable to takedown notices.
