shutter comes to my mind, I'm using it to annotate images and screenshots, it comes with a paint like edit modus (http://shutter-project.org/wp-content/uploads/

shutter comes to my mind, I'm using it to annotate images and screenshots, it comes with a paint like edit modus (http://shutter-project.org/wp-content/uploads/key_feature_062.png http://shutter-project.org/wp-content/uploads/key_feature_06...)