This was a problem in my S3 but now it is much more prevalent in my LG4. in 70% of cases, when I download a photo from the internet it is not available for sharing using fb or whatsapp, and I cannot view it from the default gallery app.
These downloaded photos can be browsed from the "Device Folders" section in Google Photos, though that sometimes fails as well to view photos.
I should mention however that EVERY downloaded photo can be browsed in the "Documents" app. I reach that app by clicking on the notification that tells me a file was downloaded successfully. There ALL the photos and files I download can be seen. I did not however find that "Documents" app in my installed apps so I have no idea how it works.