Hi Frenchy,
You can open the commandbar.htm file with notepad, then note the line for one of the commands already there, like this one:

<moi:CommandButton icon="res://deleteicon" command="delete"><moi:Text textid="Delete"></moi:CommandButton>

This part here is your icon:

icon="res://deleteicon"

If you put your icon in MoI's icon folder, you can then set that to something like this:

icon="icons/plugin.png"

Does that help?