From ArchWiki

This is a quick guide on changing the gnome "foot" icon of your main menu to the icon of your choice.

• Open the configuration editor in gnome (it should be in System Tools of your main menu) or run gconf-editor
• In the configuration editor go to apps > panel > objects > find the object for your menu (an easy way to spot the correct object is that it will have "Main Menu" in the tool tip section).
• Set the path to your icon in the "Custom_Icon" field.
• Check "Use_Custom_Icon" a little ways down.
• To see the change without having to restart X, open a terminal window and type:

killall gnome-panel