switch($this->format) {
                case 'GIF':
                    $this->oldImage = ImageCreateFromGif($this->fileName);
                    break;
                case 'JPG':
                    $this->oldImage = ImageCreateFromJpeg($this->fileName);
                    break;
                case 'PNG':
                    $this->oldImage = ImageCreateFromPng($this->fileName);
                    break;
            }
yerine

switch($this->format) {
                case 'GIF':
                    $this->oldImage = ImageCreateFromGif($this->fileName);
                    break;
                case 'JPG':
                    $this->oldImage = ImageCreateFromJpeg($this->fileName);
                    break;
                case 'PNG':
                    $this->oldImage = ImageCreateFromPng($this->fileName);
imagealphablending($this->oldImage,true);
imagesavealpha($this->oldImage,true);
                    break;
            }
png ler transparant olacaktır.