According to what I see in access logs, Bing Preview and Google Web Preview request not only the image, but all associated files from a page where the image is located - css, js and all. It was causing an extreme waste of bandwidth, invalid clicks and cookie stuffing so for awhile I just added |Preview| to the list of blocked User_agents. They got a 500 error but that's better than disabling AdSense. A visit to developers.google.com/webmasters/control-crawl-index/docs/robots_meta_tag gave me
<Files ~ "\.(png|jpe?g|gif)$">
Header set X-Robots-Tag "noarchive"
Header set Cache-Control no-cache
Header set Cache-Control no-store
Because they are not hot-linking to the images, but using a cached version in the non-visitor's browser as if BingPreview was the user's browser. When someone clicks on your image in the Image Search results page, the largest version crawled is downloaded to the searcher's browser cache, but not shown unless viewer clicks on "View Original Image" (which used to take the viewer to your site) - then they get shown the largest, highest resolution image from your site - from their browser cache and still on the image search site's "lightbox" - with no way to get to your site although it is downloaded and shown behind the image. Exact details vary between the two Image Search services, but neither is more than a scraper as far as image owners' benefits are concerned.