A bottle of water. The head of a cat. Stones. It's a warning to all cats in the neighbourhood, and to those with Internet access that may be able to view these photographs, to steer clear of places with bottled water (convenience stores excluded).
Obviously someone somewhere has had enough of cats stealing bottles of water (left out for homeless people during the hot summer months), and as a result has taken extreme action in order to deter the furry felines from causing any more annoyance.
"A cat is more intelligent than people believe, and can be taught any crime."