Howdy @Ali - You can only add image comments to "new" images. As far as I know, the D5100 doesn't give you the ability to go back to previously taken images and attach a comment.
If you're interested in enabling this feature, just hit the MENU button and highlight the 'Setup Menu'. You should see an option labeled 'Image comment'. If you check the 'Attach comment' option, the comment will be applied to all subsequent photos.
So say you're going on vacation, you could add a comment like "Family Vacation 2012" and this would get added to the metadata for all images during your trip. I hope that makes sense. Happy shooting! :)
Comments
If you're interested in enabling this feature, just hit the MENU button and highlight the 'Setup Menu'. You should see an option labeled 'Image comment'. If you check the 'Attach comment' option, the comment will be applied to all subsequent photos.
So say you're going on vacation, you could add a comment like "Family Vacation 2012" and this would get added to the metadata for all images during your trip. I hope that makes sense. Happy shooting! :)