To delete an image from a post or a page, simply click the image you want to delete and select the Delete Image button in the upper left-hand corner. This will remove the image from your post but it will still exist in your media library.
If you wish to delete the image entirely from your site, navigate to the Media Library in your sidebar. You can hover over the image and click Delete Permanently or if you wish to delete multiple images or media, you can check the checkbox beside them and in the Bulk Actions select Delete Permanently.
Clicking Apply will permanently remove this media from your site and it will not be recoverable unless you have it stored elsewhere.