Singularly attractive. The picture in question means a work of art, and the expression dates from Victorian times, when, one might speculate, paintings often were “prettier” than today. Mark Twain used a version of it: “He was pretty enough to frame.” (A Connecticut Yankee in King Arthur’s Court, 1889). Although Dialect Notes in 1909 noted that the term was often used of a piece of fruit, which presumably was good-looking enough to paint, it was and still is more often applied to human beings.