DNUI is a free WordPress plugin for delete not used image, will help you to find all in the database and all the images that are not being used/unused and give you the opportunity to clean up the database and space.

You will only delete not used/unused image, so don’t worry about delete image used.

Note: 

• Make some BACKUP of your database and the uploads folder, because for the moment the plugin will delete the information in the database and your server, they are irreversible, so if something goes wrong is better have a backup.
• Performs a lots of tests before use in post in production, try to use blogs with the same characteristics, since I have not tested for compatibility with other plugin, so you can see if will work fine.

Download the free WordPress plugin DNUI.