When a company is created from an email domain, Hubspot will automatically crawl their website and social media to try to find a company logo and populate it in the company icon field.
According to a Hubspot rep, there is no way to force Hubspot to do this if you add/change a domain after the fact. And Hubspot doesn't automatically refresh and try to do this at any interval, even when a new domain is added.
I realize this is merely a "nice to have," but it would be nice to have.