I found the following chunk of code here: http://www.wprecipes.com/wordpress-tip-detect-if-a-post-has-at-least-one-image.  If you wrap something resembling this around the stuff inside your loop, it should work.  

    $content = $post->post_content;
    $searchimages = '~<img [^>]* />~';
    preg_match_all( $searchimages, $content, $pics );
    $iNumberOfPics = count($pics[0]);
    if ( $iNumberOfPics > 0 ) {
         // Your post have one or more images.
    }

I think this just looks for images entered in the rich text editor - if you want to check for featured images as well you'll likely have to add something.  Check out [`has_post_thumbnail`][1].


  [1]: http://codex.wordpress.org/Function_Reference/has_post_thumbnail