It currently does what I call "base" pseudogrey conversion - meaning that any RGB value can only shift by 1 to give 6 tones between integer tones of greyscale (1786 total tones).
It's using html5 canvas elements - so it will only work in modern browsers that support it.
Also - it DOES NOT upload your image anywhere. It just loads it up in your browser locally and does the encoding on your own machine.
Due to browser limitations, if your image is large (there is not limit on how big it can be - just the encoding time will increase linearly) you will not see a Right-Click -> Save As... option. In that case, Right-Click -> Copy Image - then paste it from clipboard directly into GIMP.