If you want to delete synchronized images from Facebook Asset Library, just follow these steps:

1. Log into your Facebook Business Manager.
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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 Facebook Image Assets, check mark one (or more) image in the list and click on 'Delete Image'. 

Any image(s) that you select will be permanently deleted from your Facebook Image Assets. If you want to add the image(s) back again, you will need to log into Flowbox and sync the image again.