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I guessed it would just be some url variables on the end of each image, is that not how it works?

A common technique is that somewhere in the email you'll see something like:

The image filename is a UUID. The UUID is of course unique for each email sent, but the Web server is configured to serve the same image for any given UUID (after recording the UUID as an "open" into a database).

There isn't a way for an email client to be certain that the image is or isn't a tracking pixel.

Sure. But if you strip those out, then the pixel itself no longer has any value to anyone.

Or can't you just load it but with a proxy - they get to know it got opened but from a fake IP.

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