imagecopymerge(
$source_gd_image,
$overlay_gd_image,
$source_width - $overlay_width,
$source_height - $overlay_height,
0,
0,
$overlay_width,
$overlay_height,
WATERMARK_OVERLAY_OPACITY
);

Burayı alttaki gibi değiştirip dene birde

imagecopymerge(
$source_gd_image,
$overlay_gd_image,
$source_width / 2 - $overlay_width / 2,
$source_height / 2 - $overlay_height /2,
0,
0,
$overlay_width,
$overlay_height,
WATERMARK_OVERLAY_OPACITY
);