NextGEN Gallery Custom Fields was originally built for a friend. They needed a plugin to add extra information to images in their galleries, so that’s exactly what this plugin does.
You can choose between a single line ‘input’, a multi-line ‘textarea’, or create your own options and create a ‘drop down’.
Check it out at the WordPress Plugin Directory. This is where you will find the latest stable version.
If you have any questions or comments, create a post on the official forum.