Once you hit your plan's quota for properties, trail cameras or photos you will no longer be able add the item that is maxed out. To add additional resources you will need to upgrade to a higher plan or delete existing resources.
For example, if you delete 1,000 photos you will be able to add an additional 1,000 photos to your plan. Mass deleting of photos is very easy when viewing your photo gallery. Just select "Delete Photos", select the ones you want to delete (or press "Select All") and the photos will be deleted. We've put together a short video on editing trail camera photos on our blog.