show featureed image when width >= 1024px

How about this:

<!-- SET BACKGROUND -->
<?php
		// Get the image ID as a string.
    $thumb_id = get_post_meta($post->ID,'_thumbnail_id', true);
    // Get an array containing the image url, width, and height.
    $thumb = wp_get_attachment_image_src($thumb_id, false);
    // Get the width of the image.
		$width = $thumb[1];
		// Get the height of the image.
		$height = $thumb[2];
 	 if ( has_post_thumbnail() && !is_home() && !is_archive() && !in_category('discography') && $width >= 1024 ) {
    		$src = $thumb[0];
    	} else {
    		$src = get_bloginfo( 'stylesheet_directory' ) . '/img/bg/rotate.php';
    		// Use the PHP getimagesize function to get the dimensions of this image.
    		$image_info = getimagesize($src);
    		$width = $image_info[0];
    		$height = $image_info[1];
    	}
 ?>
<div id="maximage">
    <img class="siteBackground" src="<?php echo $src; ?>" width="<?php echo $width; ?>" height="<?php echo $height; ?>" alt="Site Background Image" />
</div> <!-- #maximage -->

I changed a bit of your code to make it more readable, and added the height and width for both images as attributes to the img tag so the end-users’ browser won’t have to calculate the size before rendering the page. That speeds things up. I also added the Alt attribute, since you’re supposed to always have that to validate HTML5.