Their line of sight toward the enemy position from that point is spotty, but they should have sight to some of the critical points, such as the clump of trees to the left where the gun might be, the right edge of the stone wall, and the edge of the bushes just past the crossroad. Try moving the pointer around a little more - the spots are small, but they're there.
Also, from the second pic it looks like your artillery option is grayed out. Do you have your mortars deployed nearby yet? Once you do, pull up the artillery menu and use one of the targeting options there, it might make finding the visible areas easier.