As Howie mentioned, when you're releasing skins, it's common courtesy to ask for permission if you're going to use something, i very rarely say no, the only time is if its going in a paid mod or its a full game thing taken with no changes.
@GIMN has effectively given me a blanket waiver to use his stuff in my skins, I still ask every single time, because i know the time he's put in. I'm in the early stages of an update to TCS, and i've put about 15 hours of my free time in, that's time i could spend with my partner, time i could spend playing games or watching movies i've put on the back burner.
Here's an interesting discussion about asking for permission we had a few months back