IMINT is a discipline of espionage
operations that is of relatively recent vintage. IMINT stands for IMagery INTelligence
, and involves the gleaning of operationally, strategically or politically useful information from the capture, processing and (most importantly) interpretation of photographic images.
Prior to the invention of the camera and platforms to carry cameras to the vicinity of the target, IMINT was really HUMINT, limited to human-created reproductions of scenes or images using drawing or painting tools. With the camera, however, an entirely new discipline of intelligence collection was created.
Early IMINT was acquired from high vantage points such as hilltops or hot air balloons, allowing commanders relatively real-time information on the disposition of any enemy forces or strongpoints within the camera's range. As cameras became more sophisticated and the aircraft came into general use, it became feasible to capture fairly high-quality images of rear areas, marshalling zones, transport links - any and everything that your opponent used to make modern war. Zeppelins were used for a brief time for this, being more handy than fixed balloons - Zeppelins LZ16, LZ20, LZ22, LZ24, and LZ25, for example, were used for battlefield recon in World War I However, the Zeppelin was quickly forced out of this role with the flourishing of the pursuit fighter, as Zeppelins and other airships were just too vulnerable to powered aircraft to risk near any enemy-held positions and too fragile and unwieldy to survive in general warfare. Most of these were destroyed after forced landings brought on by either equipment failure, weather, or attack by the enemy.
One problem with IMINT is that it is tempting to believe that the 'picture shows the truth.' This is not correct. The images show what the camera was able to capture, which may not precisely (or even roughly) reflect what was actually occurring at the target. Atmospheric conditions can degrade the image, making it more difficult to understand via blurring or interference. The enemy (or at least, the subject of the imaging process) can undertake all manner of deception and concealment operations in order to deny the camera a clear view of its actual target.
As technology advanced, the aircraft flew higher and faster, requiring higher and higher technology cameras to keep up. Finally, satellite imaging systems provided one of the ultimate tests of the photointerpreter's skill - the interpretation of images taken from miles above the target is something with a poor rate of success unless the interpreter is extremely experienced.
In fact, photointerpreters have a phrase to remind themselves of the difficulty of what they do and the ways that their source data can lie. The actual events, or the actual view of the target, is called ground truth - in other words, the truth of what was happening or what existed on the ground. If an interpreter has visited a target site in person and thus has personal experience with what the image they're looking at depicts, they are said to 'have ground truth.'