Custom Image Background in Block
You can do it via a class that you selectively add to the pages, then using CSS add a background. Two modules stand out:

Node Class


  Node Class is a simple module that allows users to add custom CSS
  classes to any node through the node/add interface.


Context


  Context allows you to manage contextual conditions and reactions for
  different portions of your site. You can think of each context as
  representing a "section" of your site. For each context, you can
  choose the conditions that trigger this context to be active and
  choose different aspects of Drupal that should react to this active
  context.


See also:

Is it possible to add css class to the page's body tag with rules when certain conditions are met?

and 

Context Block Classes