While the overall result is impressive, the "stitching" isn't perfect. On most pictures it's hard to tell since the brushstrokes have lower resolution than the photography. But on one picture in particular, called "O Livro (os Cem)" by Jac Leirner (1987), the stitching irregularities are easy to find. Type "O Livro" in the search box to find this image.
Essentially this picture is a giant canvas of words in Portuguese. (I speak Portuguese, and it starts off as a bunch of rambling thoughts on money and love, degenerating into what to me makes no sense).
Anyway, to pick an easy to locate spot where stitching apparently took place, find the line about 2/3 down that consists of a giant hexadecimal number (what the hell is that, anyway?). The line starts "D21D22C23..." Blow it up to maximum resolution. The first and second D, and the second 2, have alignment artifacts, and the lower portion of this starting string is slightly blurrier that the top portion. This even gives some insight into the algorithm, where you can see that it's desperately trying to align the top and bottom portions, even distorting or shifting some in-between parts to achieve the result.