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Another idea is from (amateur) astronomy: To find moving objects within astronomical pictures (of the same area) one quickly shows two figures one after the other. The minor change of the moving asteroid clearly shows up (because this typically is the only difference).

For me a similar approach would be helpful to compare two pictures, show them quickly one after the other, e.g. by pressing spacebar to immediately switch to the next picture or automatically at a selectable frequency (if display is fast enough).
One might extend that idea to display two selectable regions even of the same picture to compare color/brightness...
The advantage of showing the two pictures in sequence is that one has less constraints with monitor size resp. reduced picture size.

Since my proposal only compares two pictures the GUI might be simple, too. Select two pictures from digikam albums and press the (new) compare button. Zooming and moving can be done on each display individually, maybe with an additional sync button to show the same portion of picture1 within picture2.