Funny this subject should come up. I was just reading CS Lewis's "An Experiment in Criticism" last night. Here are some relevant excerpts:

This attitude, which was once my own, might almost be defined as 'using' pictures. While you retain this attitude you treat the picture--or rather a hasty and unconcious selection of elements in the picture--as a self-starter for certain imaginative and emotional activities of your own. In other words, you 'do things with it.' You don't lay yourself open to what it, by being in its totality the thing it is, can do to you. (pp 16-17)

If this is how the many use pictures, we must reject at oncer the haughty notion that their use is always and necessarily a vulgar and silly one. It may or may not be. The subjective activities of which they make pictures the occasion may be on all sorts of levels. To one such spectator Tintoretto's Three Graces may be merely an assistance in prurient imagination; he has used it as pornography. To another, it may be the starting-point for a meditation on Greek myth which, in its own right, is of value. It might conceivably, in its own different way, lead to something as good as th epicture itself. This may be what happened when Keats looked at a Grecian urn. If so, his use of the vase was admirable. But admirable in its own way; not admirable as an appreciation of ceramic art. The corresponding uses of pictures are extremely various and there is much to be said for many of them. There is only one thing we can say with confidence against all of them without exception: they are not essentially appreciations of pictures.

Real appreciation demand the opposite process. We must not let loose our own subjetivity upon the pictures and make them its vehicles. We must begin by laying aside as completely as we can all our own preconceptions, interests, and associations. We must make room for Botticelli's Mars and Venus, or Cimabue's Crucifixion, by emptying out our own. After the negative effort, the positive. We must use our eyes. We must look, and go on looking till wee have certainly seen exactly what is there. We sit down before the picture in order to h ave something done to us, not that we may do things with it. The first demand any work of art makes upon us is surrender. Look. Listen. Receive. Get yourself out of the way. (There is no good asking first whether the work before you deserves such a surrender, for until you have surrendered you cannot possibly find out.) (pp 18-19)

From the example of the man who uses Tintoretto as pornography it is apparent that a good work of art may be used in the wrong way. But it wil seldom yield to this treatment so easily as a bad one. Such a man will gladly turn from Tintoretto to Kirchner or photographs if no moral or cultural hypocrisy prevents him. They contain fewer irrelevancies; more ham and less fill.

But the reverse is, I believe, impossible. A bad picture cannot be enjoyed with that full and disciplined 'reception' which the few give to a good one. This was borne in upon me lately when I was waiting at a bus stop near a hoarding and found myself, for a minute or so, really looking at a poster--a picture of a man and a girl drinking beer in a public house. It would not endure the treatment. Whatever merits it had seemed to have at the first glance diminished with every second of attention. The smiles became waxwork grins. The colour was, or seemed to me, tolerably realistic, but was in no way delightful. There was nothing in the composition to satisfy the eye. The whole poster, besides being 'of' something, was not also a pleasing object. And this, I think, is what must happen to any bad picture if it is really examined. (P. 20)

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Putting all these together I'd venture the following definition: pornography is work which can be used to tittillate, but which does not hold up as good art under close scrutiny.

Alex