The forum software doesn't read that URL as a link to an image. Unlike, say, an image uploaded to Imgur that has a URL which is
only an image, that URL includes the "frame" of Google Photos information.
Contrast it with this Imgur link I used this morning:
Code:
https://i.imgur.com/x5oTQdY.jpg
I'm not familiar with Google Photos, but by right-clicking and choosing "Open link in new tab," and then right-clicking the image in that link and choosing "Open image in new tab," I was able to get an image link the forum could process. There might be a faster way.

(I'm using Chrome on a desktop PC; YMMV with other browsers/platforms.)
Code:
https://lh3.googleusercontent.com/pw/ACtC-3dclQ85uUdmxm9mOKasowK4ENc_NDIyCwzQz8p80Hr8Jc7K1vqfdC-exAAKxew-DY0QY6T2fKoqIqJy8mG_A0hclOypAMPNS3eYiuajm_9qCNVitqm5UuwAvv8_cLL0kslnnMPAhpbrAEtLwnFQCq0=w2560-h1192-no?authuser=0