Unfortunately, it seems as though different browsers will act differently. Firefox: Will send the header X-moz: prefetch whenever a page is "pre-fetched." Chrome: You're out of luck, simply because it forces you to use the Page Visibility API, which is JS-based. The term "pre-rendering" applies exclusively to Chrome, as Google seem to be doing their own ...
Simplest would be to add an image sitemap: http://googlewebmastercentral.blogspot.co.uk/2010/04/adding-images-to-your-sitemaps.html The idea of a sitemap is to help Google with uncrawlable content.
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