You could post a captured image of the skin first and ask if anyone know anything about it (creator, source, date, etc.). Then based on the responses, proceed with caution and at least be honest and up front by saying something like: I am not the originator of this yada...yada but I did modify it. Sometimes browsing through the file (uhp or whatever) one can often see the creator.
We're all for sharing and giving credit when and where it's due is always the best policy.
Let's see what ya got!