I often use a plain color background rather than a picture.   When the desktop is set to a solid color - the default metro background image is shown even when "Desktop Wallpaper" is selected.  If I choose a picture for the desktop (in the windows personalize dialog) it is mimicked in start page (metro) as expected.

I hope this is clear.  I use a solid color desktop about 50% of the time at home and about 95% of the time on my work laptop.  Helps keep distractions to a minimum.

Along those lines I also echo other suggestions for a transparent background.  That would be a nice feature enhancement.

Thanks

Andy