Has the order of the icons been changed? If I remember correctly, the first icon was Rain, now it is Shower Rain?
I cannot replace the standard icons with custom. I use the MacOS app (not web editor). I can delete a standard icon, but when I click on the + symbol, I can select a custom icon, but it does not appear in the icon set.
I’m not aware of any order changes with the icons.
If you are having difficulty changing the icon images, you can always upload individual images directly and set the opacity to: $(#WCCI#)=xx?100:0$ , where xx is the Weather Icon number. See my explanation here.