A collection of three Custom Ubuntu Menu buttons I created using the button creator at www.flamingtext.com. One Ubuntu, One Menu, and one Linux, all in PNG format. I think It goes well with the Enlightenment 23oz gtk theme here: http://gnome-look.org/content/show.php/Enlightenment+23oz+gtk?content=90485
Just use gconf-editor and choose Apps, Panel, Objects, and look for the object that has menu-object in it. Click the use_custom_icon box & edit the custom-icon key with the address where you stored the .PNG file.