$myfile=$_FILES["youimage"];
function ismyimage($myfile)
{
    if((($myfile["type"] == "image/gif") || ($myfile["type"] == "image/jpg") || ($myfile["type"] == "image/jpeg") || ($myfile["type"] == "image/png")) && ($myfile["size"] <= 2097152 /*2mb*/) ) return true; 
    else return false;
}

function upload_file($myfile)
{           

            if(ismyimage($myfile)){
                $information=getimagesize($myfile["tmp_name"]);
                $mywidth=$information[0];
                $myheight=$information[1];

                $newwidth=$mywidth;
                $newheight=$myheight;
                while(($newwidth > 600) || ($newheight > 400 )){
                        $newwidth = $newwidth-ceil($newwidth/100);
                        $newheight = $newheight-ceil($newheight/100);
                    } 

                $files=$myfile["name"];

                if($myfile["type"] == "image/gif"){
                $tmp=imagecreatetruecolor($newwidth,$newheight);
                $src=imagecreatefromgif($myfile["tmp_name"]);
                imagecopyresampled($tmp, $src, 0, 0, 0, 0, $newwidth, $newheight, $mywidth, $myheight);
                $con=imagegif($tmp, $files);
                imagedestroy($tmp);
                imagedestroy($src);
                if($con){return true;} else {return false;}
                }
                else if(($myfile["type"] == "image/jpg") || ($myfile["type"] == "image/jpeg") ) {
                $tmp=imagecreatetruecolor($newwidth,$newheight);
                $src=imagecreatefromjpeg($myfile["tmp_name"]); 
                imagecopyresampled($tmp, $src, 0, 0, 0, 0, $newwidth, $newheight, $mywidth, $myheight);
                $con=imagejpeg($tmp, $files);
                imagedestroy($tmp);
                imagedestroy($src);
                if($con){  return true;} else {return false;}
                    }
                else if($myfile["type"] == "image/png") {
                $tmp=imagecreatetruecolor($newwidth,$newheight);
                $src=imagecreatefrompng($myfile["tmp_name"]);
                imagealphablending($tmp, false);
                imagesavealpha($tmp,true);
                $transparent = imagecolorallocatealpha($tmp, 255, 255, 255, 127);
                imagefilledrectangle($tmp, 0, 0, $newwidth, $newheight, $transparent); 
                imagecopyresampled($tmp, $src, 0, 0, 0, 0, $newwidth, $newheight, $mywidth, $myheight);
                $con=imagepng($tmp, $files);
                imagedestroy($tmp);
                imagedestroy($src);
                if($con){ return true;} else {return false;}
                    }

                }
                else return false;
}

http://stackoverflow.com/questions/32243/can-png-image-transparency-be-preserved-when-using-phps-gdlib-imagecopyresample

AL subir imagen png, se deben pasar al directorio con transparencia

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