[Akula]

Some licenses only allow you to install it, not modify any content or really do anything else but install it, I don't know about the Homeworld license that's mainly why I ask.
Also yes there's the free marketing, but some companies don't care the slightest about this.
After all you're also giving material free marketing by putting it up on peer to peer, but I hardly think any companies like that, and some companies don't see the difference.
Some companies pretty much states that they won't allow people to post material, modified or not, sometimes not even for their own game. I don't think it matters if it's just a picture and such, but some will react really harsh if you convert their material for another game, especially if you don't ask for permission first.