Hi Bombbe,
Welcome to the Microsoft Q&A Platform! Thank you for asking your question here
Based on your query,
Do you want to allow only a specific image and block all remaining images from the Azure Marketplace? If that is the case, you can allow custom images by following the documentation provided below.
https://github.com/Azure/azure-policy/tree/master/samples/Compute/allowed-custom-images
you can deploy with Azure CLI, PowerShell, or JSON. Everything is mentioned in the documentation.
Try with Azure PowerShell
Try with Azure CLI
Pleases let me know if you need any further assistance or have any questions. I’m always available to support you and help resolve your issue as you needed.!
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