Additional features, settings, or issues not covered by specific Microsoft Teams categories
I accessed my Teams account online, and through inspecting with the browser tools, I discovered that this is the source of file Microsoft's generic Teams icons:
https://statics.teams.cdn.office.net/evergreen-assets/teamavatars/TeamAvatarsSpriteClassSetUp.svg
In case they move it, here's what it looks like:
The dimensions of the image are irrelevant. They use HTML to describe the specific area that is visible, so you see one icon at a time. HTML also tells it what size to be. Because it's an SVG, it can scale without pixelation.
In my company Teams environment, I can only select a JPG or PNG for a custom image. I don't know if that is a limitation my company set, or if Microsoft limits the file type.
This doesn't help solve our problems when trying to upload a custom avatar. But it does explain why MS Teams default avatar icons look good, and ours don't.