Create a watermark / Drupal 8

Install module Image Effects:

composer require drupal/image_effects ; drush en image_effects -y

Upload the watermark. I'll put it in a folder:

sites/default/files/custom/watermark/test-watermark.png

Create or edit an image style. Add a new "Watermark" effect.

Build the following settings:

Watermark drawing: Watermark file path;
Watermark resize: I only set the width, the height is calculated automatically;
Position:Center by default.

Save.

Do not forget to assign this image style to the desired image. In my case - it is a picture announcement news.

Result:

Before:

After: