Digital marketing firms have long used tracking pixels to analyze email and web marketing campaigns. These pixels are image files that are usually just one pixel in size, a design which prevents users from noticing them in most cases. They can also gather information about a user including their IP address, operating system, web browser and send it to a designated email address. The operator of that address can then use that information to fine-tune an advertising campaign. Check Point detected tracking pixels used for a phishing campaign back in August 2016.”]
Source: https://grahamcluley.com/tracking-pixels-can-conduct-surveillance-targeted-attacks/