The transparent proxy wouldn't care whether images are being dynamically loaded or not; it'll flip any image it sees (as long as it's a GIF or JPEG, in this example).
But HTTPS breaks it completely; that's kind of the point (can't MITM a TLS connection on a computer you don't control without certificate errors popping up).
The proxy seems to be matching on filenames, not content-sniffing arbitrary bytestreams for anything which might be a compressed image, so on more dynamic sites (especially SPAs), that would seem to be skipping most of the content.
But HTTPS breaks it completely; that's kind of the point (can't MITM a TLS connection on a computer you don't control without certificate errors popping up).