PNG wordt wel verkleind, alleen niet transparant, dat wordt zwart.

<?php
//Maximize script execution time
ini_set('max_execution_time', 0);

//Initial settings, Just specify Source and Destination Image folder.
$ImagesDirectory    = '../upload/'; //Source Image Directory End with Slash
$DestImagesDirectory    = '../upload/'; //Destination Image Directory End with Slash
$NewImageWidth      = 800; //New Width of Image
$NewImageHeight     = 800; // New Height of Image
$Quality        = 80; //Image Quality

//Open Source Image directory, loop through each Image and resize it.
if($dir = opendir($ImagesDirectory)){
    while(($file = readdir($dir))!== false){

        $imagePath = $ImagesDirectory.$file;
        $destPath = $DestImagesDirectory.$file;
        $checkValidImage = @getimagesize($imagePath);

        if(file_exists($imagePath) && $checkValidImage) {
			 //Continue only if 2 given parameters are true{
            //Image looks valid, resize.    
            if(resizeImage($imagePath,$destPath,$NewImageWidth,$NewImageHeight,$Quality))
            {
                echo $file.' resize Success!<br />';
                /*
                Now Image is resized, may be save information in database?
                */

            }else{
                echo $file.' resize Failed!<br />';
            }
        }
    }
    closedir($dir);
}

//Function that resizes image.
function resizeImage($SrcImage,$DestImage, $MaxWidth,$MaxHeight,$Quality)
{
    list($iWidth,$iHeight,$type)    = getimagesize($SrcImage);
    $ImageScale             = min($MaxWidth/$iWidth, $MaxHeight/$iHeight);
    $NewWidth               = ceil($ImageScale*$iWidth);
    $NewHeight              = ceil($ImageScale*$iHeight);
    $NewCanves              = imagecreatetruecolor($NewWidth, $NewHeight);

    switch(strtolower(image_type_to_mime_type($type)))
    {
        case 'image/jpeg':
            $NewImage = imagecreatefromjpeg($SrcImage);
	     break;
        case 'image/png':
			// integer representation of the color black (rgb: 0,0,0)
			$background = imagecolorallocate($SrcImage, 255, 255, 255);
  	        // removing the black from the placeholder
 	        imagecolortransparent($SrcImage, $background);
 	        // turning off alpha blending (to ensure alpha channel information is preserved, rather than removed (blending with the rest of the image in the form of black))
 	        imagealphablending($SrcImage, false);
 	        // turning on alpha channel information saving (to ensure the full range of transparency is preserved)
 	        imagesavealpha($SrcImage, true);
            $NewImage = imagecreatefrompng($SrcImage);
		    break;
        case 'image/gif':
			// integer representation of the color black (rgb: 0,0,0)
			$background = imagecolorallocate($SrcImage,  255, 255, 255);
			// removing the black from the placeholder
	        imagecolortransparent($SrcImage, $background);
            $NewImage = imagecreatefromjpeg($SrcImage);
            break;
        default:
            return false;
    }

    // Resize Image
    if(imagecopyresampled($NewCanves, $NewImage,0, 0, 0, 0, $NewWidth, $NewHeight, $iWidth, $iHeight))
    {
        // copy file
        if(imagejpeg($NewCanves,$DestImage,$Quality))
        {
            imagedestroy($NewCanves);
            return true;
        }
    }
}

?>