@DaltonST thanks for the detailed workaround: I will try to find a suitable mac osx application (maybe the Gimp?) to do it.
@BetterRed yes that's what I figured: I was trying to be a bit lazy since my column is already a yes/no column, but with added likely/maybe/unlikely values in the middle, so I was hoping to be able to continue using automatic checkmarks for yes and no, and then add custom (icon) rules for likely/maybe/unlikely: that clearly works, just looks awkward due to the different scaling algorithms. I'll use the icon resizing trick to try to match the scales for now (assuming I can't just get over the aesthetic jarring sensation and continue with the mis-matched scales!)
Thanks!