Steve Atkins answers:
Traditional open tracking is measuring image loads. That’s usually a 1x1 tracking pixel, but needn’t be. It doesn’t matter where it is in the email.
In other words, a tiny, transparent image with a unique URL for each recipient is added to every email sent. The server counts how many times each image is loaded and ties that number back to a campaign, subscriber, etc.
Matt Vernhout agreed that the tracking pixel’s location in the email code is irrelevant — unless the email gets clipped for being too long. Gmail cuts emails off after ~102kb of code, adding a “view entire message” link to load the full email. If your tracking pixel is at the bottom and your email gets clipped, opens won’t be tracked unless a recipient chooses to view the whole thing.
Nout Boctor-Smith reminded us that the tracking pixel’s placement in the email code can depend on the ESP you send from — some ESPs add it to the bottom, others to the top, and some give you the option (either through modifying code or changing a setting). If you’re using a third-party tracking pixel — that is, not provided by your ESP — you can code your email to place it wherever you’d like.