The word ideal, used in this sense, has the meaning “more perfect than in real life.” When a person says: “The ideal way to spend a summer holiday”--we know even before he utters the next words, “would be,” that he is going to tell us something that he does not expect to enjoy. It is how he would have things, if he could arrange them according to his own idea of perfection. Now this is what the artist means when he calls his picture an ideal one.