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If the image does change, your alt attribute text should change the "content" of the page. However, I would question if it helps to have these pages indexed. I assume it's your gallery page you want indexed. If that is true and your site structure allows it, I would disallow all of those individual pages from being crawled using robots.txt. But that depends on the structure of the site. You can also just add a noindex tag on the image pages. David also has a good point. (Below)