The source for this image is available.
You can enable this by turning on the Resist Fingerprinting mode, which you
can do by going to
about:config and toggling the option
Resist Fingerprinting mode is very strict and might break some websites, please report breakages to BugZilla.
This feature is currently active in 10.0a6 (Tor Browser Alpha) and should be available in Tor Browser Stable versions >10.