It depends on which Desktop Environment (or pure Window Manager) and which Debian version you're using...
If you're using GNOME in wheezy, try this:
Code:
gsettings set org.gnome.desktop.interface menus-have-icons false
For Testing/Unstable, try this:
Code:
gsettings set org.gnome.settings-daemon.plugins.xsettings overrides "{'Gtk/MenuImages': <0>}"
If you're not using GNOME (why not? It's lovely...), have a look at
lxappearance → Other → untick box marked "Show images in menus"