1. clone this repo

git clone https://github.com/walksanatora/qoi-thumbnailer-nemo

2. cd qoi-thumbnailer-nemo
3. build the apt packages

./build.sh

4. install the qoi-thumbnailer and qoiconv packages

if thumbnails do not show up try

rm -rfv ~/.cache/thumbnails
<file explorer> -q # fully quits the file explorer so it can search for new thumbnailers

and re-open your file explorer

to this stackoverflow question https://askubuntu.com/questions/1368910/how-to-create-custom-thumbnailers-for-nautilus-nemo-and-caja phoboslab for the qoi format and qoiconv program https://github.com/phoboslab/qoi