Mac was bored, he'd finished his last painting and didn't know what he wanted to paint so he was trolling around the internet looking for inspiration. He found a site where someone had done a copy of Vermeer's The Girl With a Pearl Earring and decided he could do a better job of it. I think he did.
As to the Scarlet Johansson reference, that has two sources. She starred in the movie The Girl With a Pearl Earring. But funnier to us was the reference to Scarlet in the movie St.Trinian's (2007 edition), where when the girls decide to steal this painting to get money to save St.Trinian's, the Posh Totties take one look at it and exclaim, " You're going to steal Scarlet Johansson?" Funny, funny movie.
He's now looking for something else to paint.