Dirty Dishwasher water flows back into dishwasher
This is exactly what an air gap is supposed to prevent. This is something that sits on the edge of the sink, and lies between the dishwasher and disposal, but I don't see one in your setup. This ensures that any backflow pressure won't go all the way back to the dishwasher. You can buy one at any home improvement store, but installing it might be annoying.