Stylus (Stylish without the tracking) sure is a nice interface, but you don't really need an add-on for that functionality. Create "chrome/userContent.css" within your Firefox profile directory and populate it with something like:

    @-moz-document domain(example.com) {
        img { opacity: 0.05 !important; }
    }

Note that there's some about:config flag that needs to be switched since the version that got released today.