Add base64 image to another image in PHP

I'm creating dynamic image in PHP with this:

$image = imagecreatetruecolor(708, 500);

Then I add some customization, background color, watermark text...

imagefilledrectangle($image, 0, 0, 708, 500, $whiteColor);
imagettftext($image, $fontsize - 3, 0, 400, 500 - $fontsize + 3, $infoColor, $fontfile, $info);

And finaly save the file:

imagepng($image, "test.png");

Everything works well, I get that test.png with white background, watermark, everything.

But this PHP file also receive POST values from JSON, one of them is base64 image. I know I can use something like that:

$stringImage = imagecreatefromstring(base64_decode($data->image));

But I don't know how to embed that posted image to my dynamicaly created image. Is that even possible?

Note: yes, base64 image is valid image