In order to use custom OS images in OneFzuz, the image must run the Azure VM Agent.
Building custom images can be automated using the Linux or Windows image builders for Azure.
If you have a custom Windows VHD, you should follow the Guide to prepare a VHD for Azure.
From there, rather than using Image SKUs such as
Canonical:UbuntuServer:18.04-LTS:latest, use the full resource ID to the
shared image, such as
/subscriptions/MYSUBSCRIPTION/resourceGroups/MYGROUP/providers/Microsoft.Compute/galleries/MYGALLERY/images/MYDEFINITION/versions/MYVERSION
The images must be hosted in a Shared Image Gallery. The Service Principal for the OneFuzz instance must have RBAC to the shared image gallery sufficient to deploy the images.