Some pictures online have barcodes (like some cover scans). Would it make sense for an image search engine like Google's to OCR these and then look up more on the product, in order to attach more keywords to the pic? Is that possible?

I think it would make sense.

It would also make sense to OCR ISBN numbers or Barcodes of book and to link to Google Book if available.

Or, now that you mention it, perhaps for every Google Book search details page of a book, to show the pages which include the book as cover image (based on a match with the OCR'ed barcode, as well as perhaps the title mentioned in the page, though that might be too ambiguous for some titles).

I could see something like this pull up data from sites like barcodepedia.com or similar sites.