To clean your WordPress unused images, first log in to your WordPress admin.
After this visit the WordPress media directory. For this you have to click on “Media >> Library” option.
Now all the images of your site will be visible. Here you have to click on the option “All media items” located at the top and you have to select “Unattached” option from the drop-down menu. After that click on the “Filter” button.
As soon as you click on the Filter button, all your unused images (which do not attach to any post or page) will be visible.
To delete these images, move your mouse pointer over the image you want to delete, then click "Delete Permanently".
If you want to delete these images from WordPress media library in one go, then you have to check the box that appears before File option.
Then click on Bulk Actions and click on “Delete Permanently” from the drop-down option and click on “Apply” button.