55th Siege Battery Royal Australian Garrison Artillery, Voormezeele, 15 September 1917. The 9.2" howitzer was an excellent weapon with good range and shell power. However, it was not very mobile; the large box at the right was part of the weapon's frame that helped hold it in place. Every time the howitzer was moved it had to be emptied of around nine tons of earth, disassembled, moved, reassembled, and refilled. In 1918, the rough estimate was that these howitzers were of no use if the front moved more than 3,000 yards per day, as the time taken in moving the weapons far outweighed their time in action.
These gunners, near the Ypres Salient, are presumably firing a night harassing mission. They are masked against German gas. Gas was commonly used at night because sleeping soldiers were more likely to breathe enough gas to incapacitate or kill them before they could be woken by an alarm. The British used the same logic as well.
Source: IWM photo E693.