Does a plugin exist, WPMUDEV or otherwise, that enables users to click on an area in an image and then comment on it? Imagine a picture of London, and a user can click on the Tower of London and type a comment. Then another user will see an icon where the previous user clicked, and if they click the icon it will bring up a dialog box with the previous user's comment, which can be replied to (thus inciting discussion over focal points of images). Sort of like tagging images in Facebook, except you aren't attaching a 'person' to the tag area, you are attaching a comment, which can be replied to by other users.
A 'hide annotations' button shown near the picture will remove the overlay, to prevent users becoming frustrated by pictures being blocked by the comment icons.