According to the article, the post office will share the photos with agencies that request it. I searched "mail in ballot envelope 2020" and the front of the envelope does seem to have a bar code which might match the one on the ballot. The image would reveal the address, postmark and handwriting of the sender, and in some states possibly even the name and signature.
It could prove how many mail in ballots were sent legally, which I assume is much lower than mail-in turnout.
Cannot prove how many were returned (think ballot drop boxes, everywhere).
Yes. It would invalidate the ballots brought in in suitcases and beer coolers.
According to the article, the post office will share the photos with agencies that request it. I searched "mail in ballot envelope 2020" and the front of the envelope does seem to have a bar code which might match the one on the ballot. The image would reveal the address, postmark and handwriting of the sender, and in some states possibly even the name and signature.
Here's a picture of an unused envelope:
https://duckduckgo.com/?q=mail%20in%20ballot%20envelope%202020%20georgia&iax=images&ia=images&iai=https%3A%2F%2Fbillypenn.com%2Fwp-content%2Fuploads%2F2020%2F06%2Fmailinballot-envelope-1200x630.jpg