Recently at one stock site I saw some illustrations that looked traced images. With Tineye, it was easy to find my suspicions were right. Images were from prodocts at sale online - toys, motorcycles - and even from a tattoo website. I contacted the stock site giving the links to the images, but they said this would fall on the "derivate work of art" criteria and were acceptable. Geez, even colors were the same as the original.

Is there a rule for this somewhere or is it a judgement on the sole discretion of the sites?