The Jez McKinley Wiki

In GTK3?, icons on buttons and menus were removed. To add them back into gnome, use this site:

https://gnomeshell.wordpress.com/2011/09/11/show-and-hide-the-buttons-icons/

However, in Cinnamon, this doesn't work. You also cannot set keys that don't exist. Thus to find the key, use:

gsettings list-recursively |grep icons

to set in menus:

gsettings set org.cinnamon.settings-daemon.plugins.xsettings menus-have-icons true