Select the first photo to be deleted by clicking on the circle in the upper left hand corner. Scroll down to the last photo to be deleted, then press the Shift key down and click on the upper left hand corner circle of that last photo. This will select all of the photos from beginning to end. Then press Delete and all the selected photos will be removed. If you have a lot of photos, it is easier to do this in batches (also tedious but it’s not easily designed for big deletions), especially if your internet connection is slow. Note that the thumbnails must all have loaded before selecting everything; using the scrollbar can be too quick to allow this to happen, so either do smaller batches or scroll through the images by mouse, slowly.