1. Open the Image gallery and leave it open.
2. Then open another Internet Explorer window, in there, go to your website and login as administrator (If necessary, you might already be logged in).
3. Go to the Admin, File manager.
4. Go to the Images folder and
5. Switch back and forth between the two windows now.
6. Look at the image gallery to see which files to delete and then switch back to the file manager window to delete them. (See the tip below.)

Please allow one minute to pass between deletions or you will see an error message.

Tip: check a number of them at once and then delete the group of them in order to bypass the one minute delay when deleting one-at-a-time.