I think the icons for Max's CustomUI mod are stored as embedded base64 data inside each .js file.

So you would need to edit the script files, you can find them in the moi appdata startup folder, and change the line that says:

var icon = "data:image/png;base64,iVBORw0KGgoAAAANSUhEUg <...large amount of base64 data ...>

to instead say:

var icon = "icons/LightingPresets.png";

And then that will make the button's icon come from that PNG file in the ui\icons folder.

- Michael