lucy24 - 8:27 am on Aug 25, 2013 (gmt 0)
There's an interesting quirk to most versions of image search:
When a user clicks on an image in search results they get the image in some type of lightbox, and there's an immediate request for the image file. But the image the user sees at this point is not the image google just fetched; it's an older cached version. (This is easiest to see when you've recently changed an image file.) Users don't see the actual image unless they click one more time.
The first request is always identifiable in some way, such as a referer in "blank.html" (literally) or the "rarely used" UA for some mobile apps. So you can serve up a one-pixel gif with an immediate expiration time. It will have no effect on what the user sees, but will save you a bundle in bandwidth.
Does your hotlinking routine also apply to empty referers? Probably not a good idea. Some humans don't send one; search engines never do; and malign image-scraping bots are best handled by other means.
At one time I experimented with rewriting large jpgs to smaller versions of the identical picture. This turned out disastrous because users then see the small jpg blown up to the pixel size of the large one, making it look as if you have crummy images. This is probably even worse for your site than not being indexed at all.