How to open an image created with imagecreatetruecolor() in imagick?

Question

PHP

Let's say I created $thumb = imagecreatetruecolor(). How can I open the resulting $thumb in imagick? Through Imagick::readImageFile error.
Addition:
The task is the following - there are 10 transparent png 1300*1600. I need to superimpose them sequentially on top of each other, and reduce the resulting image to a certain size.
1. Create an empty image $thumb = imagecreatetruecolor();
2. I open each png one by one with $img=imagecreatefrompng() and add it to $thumb with imagecopy();
3. Now I need to resize $thumb via $img->resizeImage();
Imagick::readImageFile($thumb) throws an error:
Message: Imagick::readimagefile(): supplied resource is not a valid stream resource