The artist, Cathy Nan Quinlan, writes:

When I placed this painting up for adoption, I asked three questions: Can you guess its name?, Can you tell me the 16th century artwork that it is pictured in? and Can you offer a speculation about why I painted it?
A friend suggested that I was making it too difficult for anyone to guess because I actually wanted to keep it. However, if that was true, I was disappointed because less than 24 hours later, someone came up with all the answers and I was forced to let it go.
The adopter offered a surprising and apt speculation about why I painted it--a better reason than my own. If anyone else would like to suggest a reason, there would be no prize, but I would be really interested to hear it.