This is an extension containing the R3-Alpha IMAGE! datatype code.
That code had a number of problems. R3-Alpha tried to unify a 2-dimensional structure with the 1-dimensional indexing idea of a series. This gave rise to various semantic ambiguities, e.g.
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What happens when you append a red pixel to a 1x1 image". Do you get an error, a new column to make a 1x2 image, or a new row for a 2x1 image?
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How does the system handle IMAGE! values that have been advanced via NEXT or FIND to positions other than the head?
Some of these questions were discussed here:
metaeducation/rebol-issues#801
There is nothing particularly remarkable about the implementation. But Ren-C has kept it compiling so R3-Alpha clients who use it could continue to do so. Also it serves as an example of the needs of a complex extension-defined datatype, so those can be taken into consideration.