Sorry, Mozilla forced me to release a new version with an incompatible Bootstrap framework and now the checkboxes are missing - I didn't look into it but they threatened to remove the addon if don't replace the version. I will try to fix it, meanwhile install v3.4.1 from here: https://addons.mozilla.org/de/firefox/addon/image-reverse-search/versions/

EDIT: I just submitted v3.4.3, this will fix it. Thanks for the hint!