As far i know, thats not how it works..
You have to use only 1 iconset IMG ,but you can change its size.
Only change the Lentgh, not the Width.
Add other icons below.., that has to be made very accurate, i think the best way is, to use Gimp, but you can also use paint3d for example as long you put it on the right position.
I am pretty sure you can find tutorials how to edit iconset IMGs..
For tutorial purposes, i took your attached img and changed its size, to show how the size should be changed..
(but i didnt add any other icons , these should be put below at the free place..)