Why not have all the icons in a folder available in the dropdown?
I tried adding them to my uploads folder and nothing showed. I altered my uploads folder in settings to default and I was able to see the custom icon names (dot.png) available to show but no image…just the name. When I picked that icon name, it showed nothing and I was unable to change it back. I had to delete the POI totally and recreate it to get the standard list back.
I also tried to trick the system by renaming dot.png to arts.png and swaping them out with filezilla and it disabled the whole icon selection. I swapped them back and it worked again.