to do this kind of work, you need to use GD library or ImageMagic
this code work with GD library
$photo_to_paste="image_to_paste.jpg"; //image 321 x 400
$white_image="white_image.jpg"; //873 x 622
$im = imagecreatefromjpeg($white_image);
$condicion = GetImageSize($photo_to_paste); // image format?
if($condicion[2] == 1) //gif
$im2 = imagecreatefromgif("$photo_to_paste");
if($condicion[2] == 2) //jpg
$im2 = imagecreatefromjpeg("$photo_to_paste");
if($condicion[2] == 3) //png
$im2 = imagecreatefrompng("$photo_to_paste");
imagecopy($im, $im2, (imagesx($im)/2)-(imagesx($im2)/2), (imagesy($im)/2)-(imagesy($im2)/2), 0, 0, imagesx($im2), imagesy($im2));
imagejpeg($im,"test4.jpg",90);
imagedestroy($im);
imagedestroy($im2);
that code will output: