Hi Gilles,
the icons are not individual images, the template uses SVG vector objects integrated and compiled into a one large font file, so to add new icons you would need to re-compile the font and convert your icon to SVG as well and then replace all the corresponding files in the template for all the fonts, the references to them. This unfortunately is not that easy, but there were many reasons why I switched from using normal PNG icons to SVGs, the advantages of this far outweigh the disadvantages.