That photo reminded me of the documentary "One Nation Under Dog". I knew the way shelters put dogs and cats to sleep was less than pleasant but I didn't know they did it in mass (as in putting many many dogs in one chamber at a time) until I watched that. My heart breaks to think of those poor confused animals in their last moments.
I watched it too and like you, did not know that either.
Nor that the shelters usually kill after 72 hours