Hey, I know the answer to this one. Well, part of it.
When the new image search first came out, I did some careful experimenting-- step by step through Image Search while concurrently asking for unrelated pages in a different browser so I could track each step in logs.
When a person using Image Search clicks on an image and gets the lightbox thing, google loads up your image and you see the request in logs-- but the human searcher doesn't see the image. They see a slightly scruffy jpg that google has already prepared and cached for the purpose.
If the user now proceeds to the next step, clicking on something within the lightbox, they then see your actual image.
You can save bandwidth by rewriting to a single-pix gif when the referer is "blank.html". Set its expiration to right away, and if the searcher really does take the next step, they'll get your real image.