Hey,
the boxes are images - just use your own images by using the "Display Preview image options" (part of your post/page editor). Just edit the pages/posts you selected to appear on the main page (under Display Options > Mainpage options) and insert the images you like. If you just want to remove the images open up index.php an search for following code:
$placeholder_image1 = get_bloginfo('template_url').'/files/medium1.jpg';
#$placeholder_image2 = get_bloginfo('template_url').'/files/medium1.jpg';
#$placeholder_image3 = get_bloginfo('template_url').'/files/medium1.jpg';
$placeholder_image1 = kriesi_build_image(array('url'=>$placeholder_image1,'height'=>'124','width'=>'280'));
#$placeholder_image2 = kriesi_build_image(array('url'=>$placeholder_image2,'height'=>'124','width'=>'280'));
#$placeholder_image3 = kriesi_build_image(array('url'=>$placeholder_image3,'height'=>'124','width'=>'280'));
and delete it.