i have a problem !

Hi Webmaster !
I had some problems as follow:
I have 2 pictures:
1- Frame (.png): was chiseled (empty inside) Transparent.
2- Picture (.jpg).
I want to design frame by PHP but when i change frame size by 2 functions
imagecopyresampled() and imagecopyresized(), My frame have a probem - A
transparent inside frame with picture.png is painted with black color and
with picture.gif is wihite color.
Please direct me to solve this problem I want to change frame size that my
frame is also in transparent status as begining. Thanks so much.

My source :
$img = imagecreatefromjpeg($cachepath.$file);
        $w = imagesx($img);
        $h = imagesy($img);
        $watermark = imagecreatefrompng($background_file);
        $watermarkwidth =  imagesx($watermark);
        $watermarkheight =  imagesy($watermark);

        $frame = imagecreatetruecolor($w, $h);
        imagecopyresampled($frame, $watermark, 0, 0, 0, 0,$w, $h,
$watermarkwidth, $watermarkheight);

        imagecopy($img, $frame,  0, 0, 0, 0, $w, $h);
        imagejpeg($img, $cachepath.$newfile, 100);

Please tutorial in a specific. Thanks so much.