There are two ways to bulk delete your media files in WordPress. Here is one way.
You will have to go through and click every single image, which can be a bit tedious if you have a lot of images.
Here is the second way. You can select a single checkbox, and all 20 media files will be automatically selected. Under “Bulk Actions”, you can deleted all selected media file. But you will have to go through each page and delete 20 files at a time.
If you are needing to delete a lot of images, you might try looking into a plugin that can mass delete many images. We have not tested any. Make sure you proceed with care.