Create a tileable background image from the left-most pixel of an image

background_image.php

<?php 

function create_background($filename) {

    // Get the file extension
    $file_pieces = explode('.', $filename);

    $file_name = $file_pieces[0];
    $file_ext = $file_pieces[1];
    $new_filename = $file_name.'_bg.'.$file_ext;

    // If the background image doesn't exist then create it
    if (!file_exists($new_filename)) {

        // Get dimensions of the original image
        list($current_width, $current_height) = getimagesize($filename);

        // The x and y coordinates on the original image where we
        // will begin cropping the image
        $left = 0;
        $top = 0;

        // This will be the final size of the image (e.g. how many pixels
        // left and down we will be going)
        $crop_width = 1;
        $crop_height = $current_height;

        // Resample the image
        $canvas = imagecreatetruecolor($crop_width, $crop_height);

        // If the file is a png
        if ($file_ext === 'png') {
            $current_image = imagecreatefrompng($filename);
            imagecopy($canvas, $current_image, 0, 0, $left, $top, $current_width, $current_height);
            imagepng($canvas, $new_filename, 0);

            // Else assume it is a jpeg
        } else {
            $current_image = imagecreatefromjpeg($filename);
            imagecopy($canvas, $current_image, 0, 0, $left, $top, $current_width, $current_height);
            imagejpeg($canvas, $new_filename, 100);
        }

    }
} 

?>

<?php create_background('your-image-filename.ext'); ?>

script.js

$(document).ready( function(){
    // Get the filename of the image and split it at the dot
    var filename = $('img').attr('src').split('.');
    // Reconstruct the filename to include the '_bg' created by the php and add the '.'
    var bg = filename[0]+'_bg.'+filename[1];
    // Apply the background image dynamically to the body
    $('body').css({'background' : 'url('+bg+') repeat-x'});
});

Requires jQuery but can be done without it.