German artillery in support of infantry, near the Narva river in Estonia; Army Group North, August 1941.

Gunners in support of infantry

The oil fields and refineries were very well defended by the Romanian and German forces, with multiple Flak batteries. The city of Ploesti and surroundings were  a significant source of oil for Nazi Germany during much of World War II.  U.S. bombers attacked it repeatedly, with more or less success. 

Ploesti storage and refiinery

Crew posing with a 17 cm Kanone 18 in Mörserlafette, probably in a training exercise.
The 17 cm Kanone 18 in Mörserlafette (17 cm Cannon 18 on Heavy Gun Carriage), was a German heavy artillery piece used during World War II. 
 It was of the "double recoil" or dual-recoil carriage, the normal recoil forces were initially taken up by a conventional recoil mechanism close to the barrel, and then by a carriage sliding along rails set inside the travelling carriage. The dual-recoil mechanism absorbed all of the recoil energy with virtually no movement upon firing, thus making for a very accurate weapon.