Fair point, I agree with this. There should probably be another icon in the browser that shows if all, some, or none of the media on a page has signatures that can be validated. Though that gets messy as well, because what is "media"? Things can be displayed in a web canvas or SVG that appears to be a regular image, when in reality it's rendered on the fly.
Security and cryptography UX is hard. Good point, thanks for bringing that up! Btw, this is kind of my field.