New answers tagged undocumented
Using FullForm, you can see that ImageSize -> 1 -> 1 is the same as ImageSize -> (1 -> 1) i.e., the option value itself is a rule. This appears to be an undocumented shorthand for scaling a graphics to the correct image size or to scale up/down by a factor. Observe the following, using a modified version of the example: img = ...
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