I know the boundaries of the arts we can or can't use... but what about arts from Kongregate?
I really want to use this one: http://www.kongregate.com/collabs/art/David3000/blazing-phoenix
The burden of proof is on you.
If you can prove that you are allowed to use the art, then you may use the art.
If you cannot prove that you are allowed to use the art or are mistaken in your proof, then you may not use the art.
Do you have proof that kongregate user David3000 is the owner of the image?
Do you have proof that kongregate user David3000 will permit you to use the image?