There's a component of Android running on your phone known as the media scanner service. This service is responsible for, in short, 'indexing' the media files on the device and providing a reference to these files to applications which request such existent media -- i.e. the Gallery or Music apps. Occasionally the media scanner does get 'confused' -- for lack of a better term -- and may skip over or ignore files for any number of reasons. Sometimes, rebooting the device will fix the issue; the media scanner performs a full indexing when the device is powered on. However, if this is not the case, you'll need to manually trigger the scanner service to re-scan the available storage. There are applications in the Play Store that will do this for you programmatically.
Here are two such applications, with no endorsements or guarantees from me.
It's likely that by using a file browser application to load, browse through, and select a file, the media scanner correctly identified and reported the image files you weren't seeing in the gallery.
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