Tom,
There seem to be two issues here. One is the count of available buttons,
the other is the resolution of the button images.
There are many, probably over a hundred, toolbar buttons in the various
categories, so I'm not sure what you mean by 42 (7x6). For any button
action outside of the available buttons, you would write (or record) your
own macro, and assign it to any button. Or create new buttons from existing
buttons, or just start with good old "Custom button," a smiley face in the
Macro category that doesn't do anything (until you've assigned it to a
macro). Then edit its image to suit. I don't know of any limit to the
number of such custom buttons.
As for the resolution, I think I must stand by my statement. If you go into
Customize (View - Toolbars - Customize), then while that dialog is open,
right click any button on a toolbar (in the Excel window), and choose "Edit
button Image," you'll see a pretty rough (16x16, as you said) rendering.
That same image looks plenty resolute enough in the normal small size of the
button.
I might be missing the point still. Please post back if I have.