Not sure if this helps ; Clarity v13 Image Display Mappings.docx
I had it in my mind that more recent versions allowed us to do it out-of-the-box (I might be wrong here), but the last time I did it I amended a PNG file* on the server (but that was just replacing an existing, unused icon rather than adding to the available ones, and obviously unsupported).
(* - \webroot\ui\uitk\images\odf.png)