Greyscale only works with tag

The “Images Greyscale” feature is only converting images that are used within an tag. Other assets like the Home page’s large banner image and [font-awesome] icons are unchanged.

Is there a way to convert all images & graphics — or even better, the entire website?— to greyscale?

Thanks in advance for your reply.

The page I need help with: [log in to see the link]