You need to resize the image manually with an external program, or you could use a dynamic image with an image.php and change the background to:
url(image.php?src=image.png&width=30&h.

source of the image.php
<?php
$image = imagecreatefrompng($_GET['src']);
list($width, $height) = getimagesize($filename);
$image = imagecreatefromjpeg($filename);
imagecopyresampled($image_p, $image, 0, 0, 0, 0, $_GET['width'], $_GET['height'], $width, $height);
header("Content-type: image/png");
imagepng($image);
?>