I tried the first 2 options - did not work. I also tried option 3 and another handful gallery apps - all inferior (advertisements, poor edit, poor display options, no move function) compared to Galaxy built-in. Last resort i did this:
Settings > Applications
- pick Gallery app
- memory, click 3dots at top right corner
- uninstall updates, revert to factory version
All blank/black photos reappeared.
This shows there is nothing wrong with our internal storage (or SD card). Neither are files corrupted. Problem lies with bugs in the new gallery software update.