Some large brand senders show a logo image as their sender-image in Outlook (in my case o365/office insier on mac), Windows 10 Mail, and I can't find any specifics on how or where Outlook pulls this image from and it's worth noting that the image shows even when images have not been loaded for a received email. I have verified that I don't have any contacts added locally, etc.

I have clients that are heavy B2B senders and I want to make sure their logo will show up in the sender-image in Outlook.

Here is a recent example from Marriott: