Gmail, Apple Mail, Outlook on the web, the new Outlook and the Gmail and Outlook phone apps play a GIF. Older versions of Outlook for Windows show only its first frame (Can I email), so the first frame of every Season is a finished picture: the lights all lit, the table already set.
Your name, title and contact details stay as text, as they are without a Season, so they can be read, copied and tapped even when pictures are turned off. The art and your dressed photo are pictures hosted online, which keeps the email itself light; they load when the email is opened, like any signature image.
Pieces that sit on the page with no background of their own, such as lights and greetings, have clean edges, so they look right on a light or a dark email page.