How to determine which custom header image is being shown

For the TwentyEleven theme, you can place this above your BODY tag in header.php. The theme will recognize that $header_image is already set, so no other mods would be necessary.

$header_image = get_header_image();
$image = basename($header_image);
$image = explode('.',$image);
$class="header-image-".$image[0];

Add the $class parameter to the body_class function to insert your header image class.

body_class($class);