Currently it is easy to track visitors and their ip address, browser, etc. on your crates at crates.io. One can reference an image in the readme of a crate which is now rendered when viewing the crate. The referenced image is then downloaded by the viewers browser, which means you can simply embed any tracking pixel.
For this reason, on GitHub all embedded images are downloaded by GitHub and the referenced image in a readme is replaced by an address to GitHub.
This issue is similar to comments #460, which proposes to proxy/cache shown badges in the sidebar (even though the title is mainly about Google Analytics). With user defined readmes this can now be more practically exploited.