Since my interpretation of the question differs from other answers posted, I think the primary topic is inter-frame video compression. It's about historical precedents for a pipeline that starts with the complete contents of frame n and frame n+1 in some representation, determines the differences between them, and communicates only the differences to a third party. That's as distinct from doing active calculation on only portions of an image because it is innately true that whatever isn't touched will be preserve — such as a video game that redraws only the moving sprites — because there are two complete frames, and their entirety is inspected to calculate a diff without a priori knowledge.
In which case, I'm going to nominate “Transform coding of image difference signals”, M R Schroeder, US Patent 3679821, 1972., which emanated from Bell Labs, as the first public reference I can find to digital image differencing; as per the name it's most heavily invested in transforming the differences for transmission, but "worthwhile bandwidth reduction" is the very second advantage it proposes in its abstract (emphasis added):
Immunity to transmission errors and worthwhile bandwidth reduction are achieved by distributing the difference signal, developed in differentially coding an image signal ... and other economies are achieved, particularly for relatively slowly varying sequences of images.
The filing date for the patent is given as the 30th of April 1970; it lists prior art but is the first thing I can find that is explicit about processing a digital signal rather than being purely analogue — in column 2, near the very bottom of the page and rolling over into column 3:
Moreover, the signals may be in analog form although preferably they are in digital form to simplify subsequent processing. Assuming for this illustrative embodiment that the signals are in digital form they are [pulse-code modulated].