This HTML5 program is mosaic processing in the palette of the PICO-8.
High-speed processing in the local processing.

http://www6.plala.or.jp/TimeTripper/html5/pico8.html

This is good !

Could use a few refinements. If you enter an image, you can also have the option of telling what size in X & Y will be the output.

For instance, I put in a picture that was 400-pixels across. What I got back was 400. However, a switch could be added to scale down to a size that PICO uses, like 128.

Also option of stretching to full 128x128 or maintaining proportion of original image.

Thank you for your message.
If you change a little program, it is also possible to change the size.
However, better to adjust the original image size is more efficient.