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.

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