Sorry to hear about this problem. You should get a row of buttons overlaid on the image to select your preferred format. If there is a specific site where you noticed this problem, perhaps there is a problem with the positioning (above, below, behind the image?). You can contact me using Github or email (see the links on the lower left side of the main page for the add-on)

(Update: version 1.5.1 pops up a little panel with some more options when the script is denied permission to read the page. Previously that just caused silent failure.).