one of my sites is made up of image galleries with different categories. I used to be ranked well on the main image category pages, but now I think google has penalized me for duplicate content because each photo page is essentially the same except for the image. Only my homepage ranks well now. Will creating a "no follow" link to each photo help? This way, my category pages will be considered unique, and google will not index the individual photo pages. I've read in a forum that google will still take into account pages even if there is a nofollow link. The thing is, so many people in forums talk out their ass, so I don't know if this is correct. So basically, I'm wondering if you think nofollow links are recommended here.
Yeah, nofollow would be good. If you can find a way to dynamically change the metatags of each photo- that might be better. In analogy, there's a similar script for Wordpress that varies the metatags of pages that normally end up with the same title and header
You might also want to try excluding the image pages off the index by modifying your robots file, by disallowing the robots to crawl that certain folder that includes all the image pages.
If you are able to share the URL it might help in giving you the right advise, but am pretty well sure that the search engine can't read images and they only do read the ALT tags and if it's not copyrighted materials it won't be penalized after all, make sure to add some unique text content on each image page and make it different than the category page.
If you don't want a page to be indexed, put a meta noindex tag in the header. Here is its format: <meta name="robots" content="noindex">