If you used to be a Windows user,you should prefer the Computer,Home,Network and Trash icons on your ubuntu gnome desktop.

Press Alt+F2 key combination,and this window pop up,type gconf-editor to start configuration editor.

icons_desktop

Navigate to apps/nautilus/desktop in next window,check the marked four checkboxes and change their icon-name as you want.
icons_desktop1

Done.
icons_desktop2