Here's an excerpt from the announcement:
Have you ever wanted to annotate your images on Omeka like you can on Flickr?
Now you can with the beta release of Omeka's Image Annotation plugin! Using an adaptation of Chris Woods' jQuery plugin, jquery-image-annotate, Omeka's new Image Annotation plugin allows users to add textual annotations to images. To add an image annotation, users select a region of the image and then attach a textual description.