This shouldn't be true any more, I just tried it out by grabbing the lh[0-9]* url for the image bytes and that won't open in an incognito browser without a Google Login. Can you share with me how to reproduce this "url copy"? If you want to send it privately just tack gmail.com onto my username.
Or you can file feedback from the photo web page and just tag me in it.
I just tried it with an lh3 url I got from chrome, opens fine in incognito. Repro easily: from /photo/ page, open image in new tab; copy address to incognito; expected: login challenge, actual: photo.
Or you can file feedback from the photo web page and just tag me in it.