Following Jay Versluis instructions, and with the assistance of cubecolor, I was able to implement a plugin to use my random header images in TwentyThirteen. It is working beautifully and I don't even look at it so I don't mess it up.
As you will appreciate, the code from Versluis uses a thumbnail to be displayed at the child theme's Custom Header Default Images section.
'header1' => array(
'url' => 'http://wp-multisites.com/w/wp-content/headers/header_01.png',
'thumbnail_url' => 'http://wp-multisites.com/w/wp-content/headers/header_01 copy.png',
'description' => __( 'Header 1', 'twentythirteen' )
What I would like to know is how can I implement some sort of function that will also use that thumbnail and automatically install it as the background for the widget areas, post titles and meta.
Any assistance is appreciated.