Hi @Maurizio Maggiora,
It is not possible to lock a specific virtual background for all users. As an administrator, you can use Control Hub to configure what options users have for applying virtual backgrounds to their meetings and calls in the Webex App. Users still have option to select None, Blur or organization-wide backgrounds. Please see below URL for more details:
https://help.webex.com/en-us/article/nw2egbw/Configure-virtual-backgrounds-for-Webex-App-users
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Thanks, Vaijanath S.