How do I assign custom icons to folders, as well as the default folders Desktop, Downloads, Music, etc.? I know that one way is to assign the properties of folders, but I want to assign icons in the same way that are assigned the default folders, so the custom icons will also appear in the Nautilus bookmarks. I've also seen this question >> custom icon in "Places" menu <<, but that is not what I seek.

UPDATE

My main intention is this:

I want the markers to display custom icons like the dolphin. I know it's possible, because the folder "Desktop", "Documents", "Music" has its own custom icons in bookmarks. How?