Great!
Content-Adaptive Image Downscaling is especially interesting, but Pixelated Image Abstraction (Gerstner et al. 2012) seems to give good results on photos (especially if one wants a pixelated result).
There are some code examples on github if that would be helpful:
-Content-Adaptive Image Downscaling (Matlab):
https://github.com/AyushRai09/Content-Adaptive-Image-Downsampling
and pseudocode:
https://johanneskopf.de/publications/downscaling/paper/pseudocode.pdf
-Pixelated Image Abstraction (Gerstner et al. 2012) (Python):
https://github.com/gskaggs/Pyxeled
Just be aware that the Content-Adaptive Image Downscaling algorithm is not designed for palette reduced results on natural images:
https://johanneskopf.de/publications/downscaling/supplementary/html/gerstner_com...