php imagecopy function adds image watermark

//Add image watermark (random position) to the background image, the background image format is jpeg, and the watermark image format is gif
function watermark($filename,$water){
//Get the width and height of the background image
list($b_w,$b_h) = getimagesize($filename);
//Get the watermark image The width and height of
list($w_w,$w_h) = getimagesize($water);
//Random starting position of the watermark image in the background image
$posX = rand (0, ($b_w-$w_w));
$posY = rand(0, ($b_h-$w_h));
//Resources for creating background images
$back = imagecreatefromjpeg($filename);
//Resources for creating watermark images
$water = imagecreatefromgif($water);
//Use the imagecopy() function Copy the watermark image to the location specified by the background image
imagecopy($back, $water, $posX, $posY, 0, 0, $w_w, $w_h);
//Save Background image with watermark image
imagejpeg($back,$filename);
imagedestroy($back);
imagedestroy($water);
}
//Output watermark image
watermark ("brophp.jpg", "logo.gif");
?>