File "ext.togetherjs.js"
line 30
Change
imageFile: '//pinklake.wmflabs.org/extensions/TogetherJS/togetherjs/images/togetherjs-logo.png',
to
imageFile: '//raw.github.com/ianb/togetherjs/master/site/images/togetherjs-logo.png',
or just copy the image into your Mediawiki folder and change the path accordingly.