To delete images in your Image Library, click on the Select Multiple button (bottom left of the screen)
Now click on the images you wish to delete, they will be highlighted when you click on them (as shown below).
Click on the Delete button to remove the images files from your Image Content Library.
Now click on the Crop button (bottom left).
Now adjust the area to be cropped, you can click and move the hightlighted area, as well as use the anchor points to increase or reduce the size of the area to be cropped.
After you have highlighted the area of the image to be kept, click on the Save button to perform the crop of the image and the area outside the highlighted box will be removed.