In an interesting interview about art and emotion on a Philosophy Bites podcast, Jesse Prinz makes a case for art being the creation of works intended to give a sense of wonder to the audience. This, for me, is as good a definition as any I have heard, working for any medium or genre. It also works as a signpost or instruction for the artist, and yardstick to measure a work of art.
There are wonderful things to see all around us; it is up to us to be receptive to them. But when we do, we are fortunate that we have a device to capture these revelations: our camera, the wonder machine.