Edit an image with php gd2 after capture from ffmpeg

Question

Hy. I'm new to ffmpeg. I want to merge two images after capture from a video. My php GD2 codes are correct. I've tried it to images that already stored in my pc. but it was not work for me when captured with ffmpeg. Here is my ffmpeg code. it's working.

$command = "ffmpeg -i $input -an -ss $duration -r 1 -vframes 1 -f mjpeg -y $output"; 
exec( $command, $out ,$ret);

And PHP GD2 codes starting after above line with,

$newimg=imagecreatefromjpeg($output);
$oldimg=imagecreatefrompng(__DIR__ .'/img/play.png');