By the way, a very useful question. No one is currently reading :) In any case, from a psychological point of view, it is really better to use a graphical form, so there are several ways to achieve this.
Solution # 1 - Put a link or link
It's easy to put a link or image tag with the correct URL (the one suggested by Shurik), but you must make Jenkins public so your recipients can see the image. Incidentally, a rather dangerous approach.
Solution # 2 - Download Image
Upload the image to a public image hosting service and complete Solution # 1.
Solution # 3 - Attach Image
The idea is to put the encoded image in a separate email section. Jenkins will generate the image (it can be accessed at the URL), so you can encode it and include it in your mail. Details and example:

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