:// Tracing vs. Copying vs. Referencing
:// Warning: If you are tracing/copying someone else's artwork without permission, this is considered plagiarism and you can be sued. Just something to keep in mind.
:// Unless the stock creator explicitly speaks against it, tracing stock is not an issue whatsoever and can be a helpful tool to learn anatomy. When doing this, you need to be honest with yourself and the community about when your talent ends and the tracing begins, however.
I've seen some dispute about these things on dA and I would like to clear some things up as I see them. I hope most will agree, though I know some will not. I will keep comments open for now because I'd love suggestions or questions. This is here as a resource for the community. Comments will become closed if people start harassing me.
But in the end...
Referencing is always okay, of course, if it's not tracing or copying. There is a distinct difference.
I used, with permission, ~Druid-Ulises's Erin Image.
Also, some further reading courtesy of dA, on using other people's images, and copy/tracing policy:
FAQ #304: Do you remove copies and trace-over art?
FAQ #257: What sort of permission do I need to use someone else's work?
FAQ #157: Can I use things created by other people in my submissions?