Hi @Bombbe
Virtual machines in the Azure Marketplace are VHD's that have been created by the publisher (Microsoft, Ubuntu, Red Hat etc) and stored in Partner owned storage account that is enabled for sharing these images with the customer. https://learn.microsoft.com/en-us/azure/marketplace/azure-vm-use-own-image
Long and short, there isn't one repository of images hosted and controlled by Microsoft, but multiple depending on who the image publisher is.
Alistair