A dog owner carrying a see-through bag of shit while walking their dog. Though they are being responsible by picking up their dog's crap, nevertheless, they are walking around with a bag of nasty shit in their hands!
'Shit bag' derives from the Latin 'shitius baggus', which in early Roman tragic theatre represented the potential to spoil a positive scene. The shit bag would be hurled at an actor at random.
The colour brown has since been associated with shit-bagging.