prova così:
Codice PHP:
$file1 = "URL IMMAGINE 1";
$file2 = "URL IMMAGINE 2";
$file3 = "URL IMMAGINE 3";
$size1 = getimagesize($file1);
$size2 = getimagesize($file2);
$size3 = getimagesize($file3);
$img1 = imagecreatefromgif($file1);
$img2 = imagecreatefromgif($file2);
$img3 = imagecreatefrompng($file3);
imagecopy($img1, $img2, $size1[0] - $size2[0], $size1[1] - $size2[1], 0, 0, $size2[0], $size2[1]);
imagecopy($img1, $img3, $size1[0] - $size3[0], $size1[1] - $size3[1], 0, 0, $size3[0], $size3[1]);
imagegif($img1, 'imag.gif', 100);