Yes 1 x 1 makes it, for all practical purpose, invisible--yes; now why in the world would anyone want to make a type of tracker invisible to an end
This post is a very short heads up explanation for a tracking tech that's been around for a long time, i. e., more of the Big Brother tech that I suspect most people are aware of but choose to ignore its presence (as if any of us had any choice, but maybe we do--we shall see about that).
A c & p: A tracking pixel is a transparent image, measuring one pixel by one pixel, that can serve as a valuable marketing tool when determining customer interest. Once imbedded on a Web page or in an email, a tracking pixel connects to a GIF file stored on your Web server. Each time the tracking pixel is viewed, it pulls the GIF file from the server, creating a logged event that lets you know exactly how many times customers accessed the page or opened the message. Setting up an email tracking pixel requires little in the way of computer expertise.
I would think that this tech is not limited to only being used by mercantile sites.
One page that I reviewed commented that it's also used to tell how long you were on a webpage, or had the email opened (presumably time spent reading the email).
The whole ball of wax is too extensive for me to even begin to accurately summarize here, but if you're concerned about your privacy, or lack thereof, all of you know how to research this topic.
That was used to hack some forum sites, a while back. If you search "krisbarteo" there're still references to it, too.
It's one of the reasons I only accept e-mails in plain text, too.