Some of the icons for applications, wich are shown on the desktop "menu" of Matchbox, are mising in the "start" menu (or rather "X" menu, lower left corner).

It seems all these missing icons do not have a valid pixmap (actual icon PNG file), at least on the deskto they are all shown as a toothed wheel.
How can I add them to the X menu as well (becasue I use it often when the desktop is not visible)?

And another question:
Directly besides the "X" menu there is the rxvt icon in the taskbar.
How can I change it to call mrxvt instead of rxvt?