If the photos are contained in the photos layer in the bottom left of your screen, make sure they are visible using the toggle, select the photo and the delete icon is at the top of the screen. To multi-select hold down the shift key or to select all right click on a photo and select the select feature type functionality.

If photos have been linked to a plan and you want to delete this make sure the plan is visible. Select the photo icon and the delete icon is at the top of the screen. This can be done in bulk by either holding down shift to multi-select or the select feature type function.