A tracking pixel is a small piece of the web page that's requested from the tracking domain to insert the cookie. 

The tracking pixel is usually invisible to the user on the page. Which one of these would be a good candidate?

<img height="0" width="0" alt="" src="http://track.com"/><img height="50" width="50" alt="" src="http://track.com"/>

Correct. This img tag would be invisible to the user, but would still send a request to the tracking server and store the tracking cookie. 

Would that really be invisible to the user?