On the saving schematic files page (https://github.com/Lunatrius/Schematica/wiki/Saving-schematic-files) it says you can add an icon to the schematic when saving. I've tried the method described and it just results in the usual grass block icon.

Has this feature changed or is it no longer available?

Versions:
Minecraft - 1.12.2
Forge - 14.23.4.2703
LunatriusCore - 1.2.0.42
Schematica - 1.8.0.169

It's possible that I broke this feature when I implemented support for the structure block format; however, I vaguely remember trying to use it and being unable to actually use it so it's also possible that the feature just hasn't worked in a while (that wiki page was last edited May 2015, i.e. 3 years ago).

That's fair enough, would be a cool addition either in the described form or if possible using a image in the schematics folder with the same filename so it had a bit of flexibility