Innocent plagiarising happens frequently on the Internet, especially with visual material. Who hasn't taken photographs for school projects, professional presentations or for family occasions that can now be seen on the web?
This evokes the "inadvertent plagiarism" concept advanced by the folks at SIU in the Poshard matter.
The post on "The Fourth Bit" included the text:
This is what happened to this year's winner of the San Jorge poster competition in Caceres who took the image of a dragon from a free web without knowing that it belonged to a famous illustrator. The winner considers that there was no plagiarism because he worked on the image. Collage as a creative method ends up blurring the borders between the original and the creative composition. Is the poster original or a simple plagiarism? How would it have been regarded if the true origin of the dragon were known?
This sort of thing implicates copyright rights in derivative works, as mentioned in the U.S. in 14 USC 106. One can find a relevant discussion concerning derivatives of the Mona Lisa (La Joconde), showing that the issue came up long before the internet, collage culture, or mashups.
And, recall the words in the Harvard Business Review: Plagiarize with pride.
Cross-reference: L.H.O.O.Q. (French slang -- "Elle a chaud au cul.")