Yeah, we still don’t have any firm guidelines on that. I try to either give attribution to the original if it is something I found online and i can find the source, or ask for permission to use it in this way. (The latter has not yielded anything useful to date, so there are a couple of things that I can show pictures of what I made, but can’t share the file or the link, because the authors never got back to me.)
It should be noted that the only ones I do not link to are sites that are easily copied…rather than have them see copies of their work everywhere they look, I ask for permission to post the link…some of them are really fabulous, but I can’t show them off for fear of too much copying.
Anyway, it’s probably best to only use your own work for stuff you give away or sell. I’ve actually removed a couple of items from the Free Files section for that reason.