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Thanks guys... here's the reason: A lot of people come to my site from google image searches. They typically just click on the full sized view of the image and leave. I'd rather they stick around. So I don't want them to be able to click from google image search to see the full sized image. I'd rather they be redirected to a particular page....that invites them to do other things.
Surely Google images shows the image, and also the page in which it appears. If people take one look at your image and leave, I suggest the most likely reason is that they aren't interested in the site. If you are searching for something, you typically find many sites which may be very worthy, but aren't what you are looking for - when I meet trickery intended to make sure I look at what the siteowner thinks I ought to want to look at, I generally get annoyed, and leave even faster.
Also, I imagine that if google says "Here is an image", but a person attempting to access the image actually gets something else, this would be instant reason for total deletion from Google images (rightly so).
I have a small problem with hot-linking, in that people are using pictures from puzzles to decorate their blogs and things, and the pictures aren't mine. So (when I get round to adding higher resolution images) I intend to use dynamically generated urls, which will only be valid for 24 hours or so.
(I'm not familiar with Apache rewrite rules, so I can't see quite what's going on in the various suggestions..., but it seems to me that anything based on refer*left-bracket-damnit*r*right-bracker-damnit*ers is not terribly reliable...)