This takes a folder of images and creates new images suitable for Kindle wallpapers. It'll add text to the image too.

kindle-wallpaper.php

<?php
/*
 * This finds all jpegs in a directory ($sourcedirectory) and creates
 * an advert for each one. It will:
 *  - resize
 *  - convert to greyscale
 *  - add a textbox at the bottom with a header and multiple lines of
 *    smaller texts, supplied as two arguments on the command line. The
 *    second argument may contain \n to create line breaks.
 */

/*
 * The following comments are from Yifan Lu:
 *
 * You must follow these three guidelines or your screensaver will NOT work:
 * 
 * 1) Each image MUST be a grayscale PNG that is 600 × 800
 * 2) Each image MUST be named bg_xsmall_ss##.png, where ## is a two digit number from 00 to 99
 * 3) You MUST have an image named bg_xsmall_ss00.png and you CANNOT skip a number (ex: bg_xsmall_ss00.png, bg_xsmall_ss02.png but no bg_xsmall_ss01.png)
 */

$sourcedirectory = getenv('HOME') . DIRECTORY_SEPARATOR . 'kindle-pictures';
$resultdirectory = getenv('HOME') . DIRECTORY_SEPARATOR . 'kindle-wallpapers';

delete_directory($resultdirectory);
mkdir($resultdirectory);

$sourcefilepaths = glob($sourcedirectory . DIRECTORY_SEPARATOR . '*.{jpg,JPG}', GLOB_BRACE);
$imageindex = 0;

foreach ($sourcefilepaths as $sourcefilepath) {
    $sourcefilename = basename($sourcefilepath);
    $destinationfilename = $resultdirectory . DIRECTORY_SEPARATOR . 'bg_xsmall_ss' . sprintf('%02d', $imageindex) . '.png';
    create_advert($argv[1], $argv[2], $sourcedirectory . DIRECTORY_SEPARATOR . $sourcefilename, $destinationfilename);
    $imageindex = $imageindex + 1;
}

function delete_directory($dir) {
    if (!file_exists($dir)) return true;
    if (!is_dir($dir)) return unlink($dir);
    foreach (scandir($dir) as $item) {
        if ($item == '.' || $item == '..') continue;
        if (!delete_directory($dir.DIRECTORY_SEPARATOR.$item)) return false;
    }
    return rmdir($dir);
}

function create_advert($header, $alllines, $sourcefilename, $destinationfilename) {
    $lines = explode("\\n", $alllines);
    
    $requiredwidth = 600;
    $requiredheight = 800;

    // find width and height of source
    $imagesize = getimagesize($sourcefilename);
    list($imagewidth, $imageheight) = $imagesize;

    if ($imagesize) {
        // load the image
        $image = @imagecreatefromjpeg($sourcefilename);
        
        // find the rotation exif data, and rotate the image if necessary
        $exif = exif_read_data($sourcefilename);
        $ort = $exif['Orientation'];

        // TODO: image flipping... see http://php.net/manual/en/function.imagecopy.php
        switch($ort)
        {
            case 1: // nothing
            break;

            case 2: // horizontal flip
                // TODO: $image->flipImage($image,1);
            break;
                                    
            case 3: // 180 rotate left
                $image = imagerotate($image, 180, 0);
            break;
                        
            case 4: // vertical flip
                // TODO: $image->flipImage($image,2);
            break;
                    
            case 5: // vertical flip + 90 rotate right
                // TODO: $image->flipImage($image, 2);
                $image = imagerotate($image, -90, 0);

                $temp = $imagewidth;
                $imagewidth = $imageheight;
                $imageheight = $temp;
            break;
                    
            case 6: // 90 rotate right
                $image = imagerotate($image, -90, 0);

                $temp = $imagewidth;
                $imagewidth = $imageheight;
                $imageheight = $temp;
            break;
                    
            case 7: // horizontal flip + 90 rotate right
                // TODO: $image->flipImage($image,1);    
                $image = imagerotate($image, -90, 0);

                $temp = $imagewidth;
                $imagewidth = $imageheight;
                $imageheight = $temp;
            break;
                    
            case 8:    // 90 rotate left
                $image = imagerotate($image, 90, 0);

                $temp = $imagewidth;
                $imagewidth = $imageheight;
                $imageheight = $temp;
            break;
        }

        // find how much to scale by, preserving aspect ratio
        $scalewidth = $requiredwidth / $imagewidth;
        $scaleheight = $requiredheight / $imageheight;
        $scale = min($scalewidth, $scaleheight);
        $otherscale = max($scalewidth, $scaleheight);

        // if we need to crop the original first, before scaling, this is required crop size
        $cropwidth = $requiredwidth / $otherscale;
        $cropheight = $requiredheight / $otherscale;

        // crop
        $cropped = imagecreatetruecolor($cropwidth, $cropheight);
        imagecopy($cropped, $image, 0, 0, ($imagewidth - $cropwidth) / 2, ($imageheight - $cropheight) /2, $cropwidth, $cropheight);
        imagedestroy($image);
        $image = $cropped;

        // resize
        $resized = imagecreatetruecolor($requiredwidth, $requiredheight);
        imagecopyresized($resized, $image, 0, 0, 0, 0, $requiredwidth, $requiredheight, $cropwidth, $cropheight);
        imagedestroy($image);
        $image = $resized;
       
        // convert to greyscale and brighten a little
        imagefilter($image, IMG_FILTER_GRAYSCALE);
        imagefilter($image, IMG_FILTER_BRIGHTNESS, 10);
        
        // text
        $font = '/System/Library/Fonts/LucidaGrande.ttc';
        $headerfontsize = 18;
        $fontsize = 12;
        $headergap = 6;
        $linegap = 4;

        $headerdims = imagettfbbox($headerfontsize, 0, $font, $header);
        $headerheight = $headerdims[1] - $headerdims[7];
        $headerwidth = $headerdims[2] - $headerdims[0];
        $maxwidth = $headerwidth + 8;
        $totalheight = (2 * $linegap) + $headerheight + $headergap;
        for ($lineindex=0; $lineindex<count($lines); $lineindex++) {
            $dims = imagettfbbox($fontsize, 0, $font, $lines[$lineindex]);
            $linewidth = $dims[2] - $dims[0];
            $maxwidth = max($maxwidth, $linewidth + 8);
            $totalheight += ($dims[1] - $dims[7]) + $linegap;
        }
        
        $bgcolor = imagecolorallocate($image, 31, 31, 31);
        $fontcolor = imagecolorallocate($image, 255, 255, 255);

        // background first
        $backgroundx1 = ($requiredwidth - $maxwidth)/2;
        $backgroundy1 = ($requiredheight - $totalheight);
        $backgroundx2 = $maxwidth + $backgroundx1;
        $backgroundy2 = $requiredheight;
        imagefilledrectangle($image, $backgroundx1 - $linegap, $backgroundy1 - $linegap, $backgroundx2 + $linegap, $backgroundy2 + $linegap, $bgcolor);
        imagerectangle($image, $backgroundx1 - $linegap, $backgroundy1 - $linegap, $backgroundx2 + $linegap, $backgroundy2 + $linegap, $fontcolor);

        $y = $requiredheight - $linegap;
        for ($lineindex=count($lines)-1; $lineindex>=0; $lineindex--) {
            // create a bounding box for the text
            $dims = imagettfbbox($fontsize, 0, $font, $lines[$lineindex]);
            $width = $dims[2] - $dims[0];
            $height = $dims[1] - $dims[7];
            $x = ($requiredwidth - $width) / 2; 
            $y -= $linegap;

            imagettftext($image, $fontsize, 0, $x, $y, $fontcolor, $font, $lines[$lineindex]);

            $y -= $height;
        }

        {
            $dims = imagettfbbox($headerfontsize, 0, $font, $header);
            $width = $dims[2] - $dims[0];
            $height = $dims[1] - $dims[7];
            $x = ($requiredwidth - $width) / 2; 
            $y -= $headergap;

            imagettftext($image, $headerfontsize, 0, $x, $y, $fontcolor, $font, $header);
        }

        // create directory, write image
        imagepng($image, $destinationfilename);

        imagedestroy($image);
    }
}
?>