I certainly did not expect the down votes. I've designed a number psychology experiments and I and my colleagues were always careful to balance experimental stimuli and conditions on a number of factors, ranging from luminosity to semantic association, and especially order.

When I was tasked to create stimuli or to take measurements on a series of items it was always important to try to eliminate systematic differences on non-interest, most typically thru randomization of the order of creation or measurements.

I would point out that the study which appears to be the closest thing to a tiny seed of truth to the tank story (Kanal & Randall 1964) actually did balance its photographs by construction, by subcropping large aerial photographs to have tank/no-tank sections and training on that. That necessarily controls for time-of-day, weather, luminosity etc.