That’s actually a pretty common practice.
Due security reasons, you’re not able to do an Ajax request to a third-party domain. And you really have no choice but sending data by loading an image OR loading JavaScript code which is actually server-side code. Or you can do both (as Google Analytics works, they just load JavaScript which handles all events on the page and register them by loading a pixel image with analytics parameters passed).
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solved Why does Facebook uses the img tag with PHP link [closed]