your theme might be using body_class() which gives specific css classes for each page, for instance: .page-id-123 with the page ID as numbers;
in style.css, use it for example:
body.page-id-123 { background: url(images/page123backgr.jpg); }
body.page-id-159 { background: url(images/page159backgr.jpg); }
or add conditional styles into the head section of your theme (in header.php);
example:
<style type="text/css">
<?php if( is_page(123) ) : ?>
body.page-id-123 { background: url(<?php echo get_stylesheet_directory_uri(); ?>/images/page123backgr.jpg); }
<?php elseif( is_page(159) ) : ?>
body.page-id-159 { background: url(<?php echo get_stylesheet_directory_uri(); ?>/images/page159backgr.jpg); }
<?php endif; ?>
</style>
details depend on the currently used theme.
