Note: This is applicable to Jenkins 2.222.1 and Embeddable Build Status Plugin 2.0.3. Perhaps it works for previous versions as well.
Note that the link from the markdown can be omitted, so only the badge image gets displayed.
As the final result, you should see the following.
GitHub is doing an image link anonymization, which means images are being prefetched, cached, and not served directly from Jenkins. As GitHub performs a strict SSL verification, it will fail if the certificate is invalid or your Jenkins JKS does not contain a full certificate chain. In that case, you have to recreate it.
After you fix the certificate issue, make sure to purge Camo’s cache. A Camo link can be obtained from the README.md preview mode or by simply inspecting the HTML from the browser.