Testing theory about Google photo accounts migrated from Picasa. Theory is links from migrated accounts will work long term whereas links from non-migrated accounts will stop working after some time.
This is a photo from a migrated account although this photo was added to the account after migration:
This is the same photo from a newer account not migrated from Picasa:
BTW both photo links ended in "... s1874-no" and changing that to "... s900-no" made the size better on the screen.