How were Jerome and George captured in the picture with their feet in the air?

Basically, the boys were in the process of moving through a lock when they noticed the photographer setting up. Ignoring their responsibilities, they become preoccupied with posing for the picture, This results in the bow of the boat becoming stuck in the lock gate as the water level rises. The sudden release of the boat from under the lock gate knocks them over at the exact moment that the photograph is taken.

In case you didn't know - a lock is an enclosed section of river fitted with pumps and taps, allowing the water level to be lowered or raised to provide access through the gate. Commonly used to link canals to natural waterways, sections of waterways at different levels or to simply control traffic flow.