You certainly can make an image from a Virtual Machine. The entire process is described here. Also look at my other SO answer about a caveat when running sysprep (regarding not letting the VM image shut down after sysprep runs).
Once you have your image, it will be available in your Virtual Machine gallery, where you can choose it as your baseline for new VMs (either via portal or PowerShell / CLI).
If, in the future, you do updates on your master image, you'd need to re-create an image for future deployments. Or... you'd need to install the updates on each deployed VM instance.
Note: With web/worker role instances (which doesn't help for SharePoint but could help with other workloads you have), the Windows Server OS image is maintained by Microsoft. Should there be an OS update, these are taken care of for you.