If you want to delete synchronized images from Facebook Asset Library, just follow these steps.
1. Log in to your Facebook Business Manager.
2. Open the drop down menu (1) > click on "All Tools" (2) > under Assets, click on "Images" (3) to access Facebook Image Assets.
3. Once in Facebbook Image Assets - check mark one (or more) image in the list and click on "Delete Image".
The image will be permanently deleted from your Facebook Image Assets.