Usando a função list_icons ():
icons=icon_theme.list_icons()
for icon in icons:
if 'usb' in icon:
print icon
e a saída:
drive-removable-media-usb
usbpendrive_unmount
gnome-dev-removable-usb
usb-creator
drive-harddisk-usb
gnome-dev-harddisk-usb
gnome-dev-unknown-usb
usb-creator-gtk
drive-harddisk-usb-symbolic
Agora tire sua foto