Feel free to use Sad Panda, but please just give credit to me somewhere.
I appreciate you asking instead of just ripping it off. That's a good quality you don't see all that often anymore.
As for licensing advice, it really depends on the artist and how they want their art used. You already know the most important part, ask first. Beyond that, just respect the artist's requests.
Where it comes to licensing your own work, well I don't have much to advise on that, I haven't had much experience there.