I have a desktop and a mobile site and I use rel=canonical and rel=alternate as recommended by google to link the corresponding html based pages of each site together.I'm just wondering, because on the mobile site, I'm serving smaller sized versions of the exact same image that I serve on the desktop site (requested image filenames for each site are different), I'm just curious if I get any real SEO benefit if I used the rel=alternate and rel=canonical in the HTTP headers for each desktop site image and corresponding mobile site image, or should I take it as if it is just a waste of a few bytes of bandwidth?
Does rel=canonical and rel=alternate make sense on pictures?
url;images;canonical url;rel canonical;rel alternate
As Google don't appear to mention canonical images in their guidelines, I'd assume it is not going to be an SEO issue and not bother.  The worst that I've seen happen to a page when no canonical tag is set is simply that Google pick the wrong URL as canonical and push the other one down the listings.  If images were treated in a similar fashion and they indexed your small images and pushed the larger ones down the image listings, then people finding your site via Google Images would still be taken to the same page on your site.Since Google Images allow people to search by things like size, it probably has benefit having both sizes indexable. However, if thumbnails being listed is a problem for you, then I'd personally put them in their own folder and use a noindex robots file to block the lot.