See the original idea by qdebode 
 https://planet4.greenpeace.org/idea/digital-cleanup-tool/

In the spirit of cleaning up digitally, it would be useful to have a sort of tool that spots if an image isn't being used anywhere on the site. Over time, so many images get uploaded and lost. If there was a tool to see that images aren't being used anywhere, admins could remove them and make the site a bit more green.