It is a picture of a sacrificial ceremony when Zhuge Liang was about to return to Shu after defeating Meng Ho (capturing and releasing him 7 times.)
Hs army stopped at the river when the tide was too strong for anyone to cross. Meng Ho suggested Zhuge Liang to make the sacrificial ceremony. To avoid killing people, he decided to make a substitute by making "Mantou", a human-head-shaped bun filled with beef and goat meat.
Manjus originated from China, where they are called "Mantou". Like #5 said, at "濾水" (the river) there was a traditional ceremony where you offer human heads for a safe crossing. Zhuge Liang offered mantou to the river instead so the troops can cross. "Mantou" in chinese also sounds like "瞞頭", which means "lying to cover up that it isn't really a head". Rumors conflict on whether the bun is actually shaped into a head or not.