This plugin tracks the real usage of your media and display this statistics in a simple way in the Media Library. This is useful to discover which images (or sized images) aren’t viewed in your WordPress at all.
On WordPress: Media Usage
In the Media Library, a new column “Usage” is added. In it, all your sized images for a given media are shown in the form of a little square with a letter assigned (T = Thumbnail, M = Medium, etc) and a color.
You can hover on this little square to get more information such as what it is, the number of visits and views, and also when the image was seen for the first and last time.