Since I've been looking into Austro-Hungarian fortress artillery, I've found a few sites with images and diagrams of the guns used. It's been really hard to find these (apparently googling "12 cm minimalschartenkanone" gets a lot of image results), so I'll link them here. It's best to use Google Translate for them, they're not in English.
This page (German) has information on the fortress artillery of Austria-Hungary. Click on the "AUSÜSTUNG/BEWAFFNUNG" button at the upper left, and then on the "Guns" button on the left under Armament. Also, the buttons for "truck" and "train special vehicles" above the Armament section have photos of the various trucks and artillery tractors used by the Austro-Hungarian army, which I have not seen anywhere else.
This page (Italian) has information on the fortress artillery and turret types of both Italy and Austria-Hungary from 1866 to Austria-Hungary, and almost every link goes to either a diagram of the artillery or turrets, or to a page about a fortress with more diagrams and images on how it was laid out.
This page is a PDF so I can't put a translation link, but it is in Polish, and has photos and drawings of the Przemyśl fortresses and their artillery. It's the only image I can find on the 8 cm Senkpanzerkanone M.2 (an 80 mm gun in a retractable armored turret- that's presumably what "Senkpanzerkanone" means).
This 10-page thread (Bosnian) has a huge amount of images of Austro-Hungarian coastal artillery (which is great, but not the focus of this forum), but also images of the 42 cm M.14 howitzer in its original coast defense role. These are the only images I've seen of the 42 cm howitzer in its armored turret (including several images during construction), and a few weapons shown are probably common to Austria-Hungary's fortresses.