I think it goes even further than what the previous comment said. The value of the painting is different for everyone and even for the same person it will change in different circumstances (value of a cup of water at home or in the desert).

When a transaction happens the two people very probably don't agree on the value. Rather, the seller values the painting at "less or equal" of the strike price and the buyer values it at "equal or more". The transaction happens if those two regions overlap somewhat.