I was on the Lunar Chronicles Tumblr page and there was some fanart that was submitted anonymously. Who should I give the credit to; should I just say it was found on the tumblr page? Or should I even upload it at all?
Hiya, I'd suggest you to download the image and then use Google Search (search by upload function) to find where else it is on the internet. Try to locate the original author, since the author probably posted it somewhere (like his blog, website or deviantart profile). Also, you can look if the author's name or signature is somewhere on the image. Like the first image in the Tumblr page you've linked me to says "Julie Crowell" at the left bottom. Then try to search for the author's website. If you really can't find the author, let me know. I'll also try to help and if I can't find it either, then we'd better not upload it here.
The second one had zero search results when I looked up the photo, but I found that the first had many reposts on various social media sites. I searched any instagram and tumblr pages, but there wasn't any account. The only places else I could think of searching is Pinterest, yet I unfortunately do not have a Pinterest account.