On Picasa Web Album, I can search all public photos via the explore features, for instance a search by tag: https://picasaweb.google.com/lh/view?feat=tags&psc=G&filter=0&tags=wikimania
(Note, Google is hiding such features more and more in an attempt to force people to use other services. If at some point in the URL gets redirected elsewhere, see how it looks currently.)
How do I do the same on Google Photos? https://photos.google.com/search/wikimania only finds my own photos with the word.
Google Plus doesn't provide a side access either: https://plus.google.com/s/wikimania/photos only finds (very few) posts with photos, while I only want the photos themselves and all of them.