Sometimes a connectivity issue or corrupted file transfer can cause photos to be missing after you "Saved to Cloud" from the mobile app. Here are a few steps to try:
Sometimes your browser attempts to cache pages, so let's rule out caching. Hard-refresh the page (cmd/ctrl-shift-R on Chrome/Firefox, cmd/ctrl-R on Safari, ctrl-F5 on IE) and see if the photos show up.
If the previous "Save to Cloud" did not complete, find a better Internet connection and try the "Save to Cloud" again.
If the save completed but photos are still missing you can pop into the inspection on the mobile app and tap "More Options" at the bottom. Then tap "Force Upload All Data", which will re-upload everything. (Note that this will overwrite changes done since your last Save to Cloud.)
If they still aren't present, then the image file(s) may have been corrupted. You'll need to re-add the photos either on the mobile app or on the web from your mobile photo gallery backups.