Preferred sources/methods for VirtualBox images on macOS 10.12? I have a 2016 MacBook Pro running macOS 10.12 and I want to run a Linux VM such as Mint using VirtualBox. I am looking for best practices or official/recommended sources for an image that is packaged with the macOS host OS in mind.

I have downloaded VirtualBox, and first looked for macOS-friendly pre-built images. Top web searches lead to sources like osboxes.org, however their VDI images are .7z-compressed. macOS doesn't come with a 7z decompressor out-of-the-box. I could install one (The Unarchiver, Keka, etc.), however the macOS-unfriendly compression makes me wonder whether I'm looking at the wrong sources, and if there are other macOS-friendly sources for Linux virtual images (or whether there is another process that I should be following).

What are the preferred sources/methods for installing VirtualBox images on macOS 10.12?
I would say that in-general, the preferred method of getting a virtual machine image would be to create it yourself by installing the operating system fresh from the distribution's installer. I would recommend doing this anyway, it ensures the image you get has not been tampered with by a malicious actor.

Oracle does offer Pre-Built Developer VMs. Any other sources are unofficial.