After clicking on an image that has already been inserted into your Page or Post, a small toolbar will appear over the image. Clicking the Remove button (Delete image) will remove the image from your content. It will only remove the image from your Page or Post, it will not delete the image from your Media Library.
If you wish to permanently delete an image, go to the Media Library. You can click the List View icon () to display your images in a list, or you can click the Grid View icon () to view your images in a Grid layout. If you are in Grid View, click on the image and select Delete Permanently on the lower right. If you wish to delete multiple images, select List View then check each image you wish to delete. Click on the Bulk Actions drop-down menu and choose Delete Permanently then click on Apply.