Thanks for improving - I often just reply to get the topic back on top for a while because everything here goes down out of sight very fast due to the "permanent deletors". So basically due to Google who still does not understand that the trash on mobile should be secured against complete emptying. 

PS: do you have an explanation while "my long link" will not work after some time anymore - assuming that my pictures stays in my library forever.

@Rinus Alewijnse

The photo link gotten while logged in will be invalid after a while since it is temporary, and will ask anyone who access to the link to log into a Google account after it was invalid. But viewer cannot see it even if logged in. Though I'm not sure why temporary, I think it may be the reason in relation to server maintenance.

The photo added in a shared album is already in publish state and album link is fixed, so that direct link gotten by user in logging out is also in publish and fixed. It is natural that anyone can access to it without logged in, and it is important for viewing on the website.

The link you posted is already dead. You can understand whether it is temporary link or not if you wait a few days after creating a new link by yourself.

@jeff j

Are you not capable of reading the recommended answer I wrote to tell how can a permanent link be created?

Then I only know about a very weird method via https://www.google.com/settings/takeout for Photos. 

If you do select the "album" with the date of the picture shown on Photos, you will get a json-metadata file (textfile) in the download (after unzipping) which contains a permanent link. LIke shown below: 

you can just get the file with your server and then send it to the response on the server side, then access something like "www.myserver.com/picture1", which would automatically get the google photo and append it to the server response, so the client doesnt directly see the google photo link, although if they had the link they could still get to it