Unfortunately this issue comes up pretty often in modding forums the world over, and the correct answer is that you must have the permission of the original content's owner.
If the owner finds out about an infringement and asks us to take it down, we will do so immediately and will err on the side of caution so as to reduce the potential liability to Fractal Softworks.
On the bright side, it's usually not an issue for projects that aren't-for-profit; i.e., enforcement is generally nil. But there can be only one correct, official answer to your original question. Also, most content owners (video game studios, artists, authors, et cetera) are usually pretty good-natured about getting additional free visibility for their work, and will usually merely require (as sproginator suggests) that you give full credit on the original post and/or download page for your mod so that it's clear who the original content belonged to.
If you feel intimidated about trying to acquire said permission, feel free to contact me privately and I might be able to help.
