Restart the phone. (Or run an app like
media.Re.Scan. (The phone only "sees" pictures that have been found by Media Scan - so that the entire phone doesn't have to be scanned every time you run a picture app. Uploading a picture to Google Photos doesn't trigger a rescan. [Neither does copying files to your phone.] So you can either restart the phone, triggering a scan, or you can trigger one manually with an app like media.Re.Scan.)
The same holds true for videos, music, any media files. They're all scanned by Media Scan, then it builds a database of all media files on the phone, so any app that uses any media files can just look in there and see what's available. (Phones are really fast enough these days to redo the scan every time you run a media app, but back in the days of Donut and Eclair, that would bring a phone to its knees. And in line with Google's "any app should be up and running as quickly as possible" design philosophy, they haven't changed it.)