Recover photos from thumbnails in Photos app When I open some of my pictures in Photos app, an error message occurs: "the volume cannot be found".

This is because the pictures were "imported" from the desktop on a previous computer and Photos didn't make a copy of them (the setting was disabled).

These original files were lost, however Photos is still able to display a large preview (which actually doesn't seem smaller than the original picture).

Where can I find the picture files behind Photos's previews, so I can reimport them correctly?

I've tried finding a preview file location field in Photos's sqlite database (Photos Library.photoslibrary/database/photos.db) but with no luck.

EDIT : I am technically savvy and can use the Terminal for file search, querying a sqlite database like the one mentioned above, or other tasks.
The photos are stored in:

~/Pictures/Photos Library.photoslibrary/Masters/


To get there go to your Pictures folder in your users directory and right click on your photo library file. Then right click and select  “show package contents”  I’ve seen the same photo in a variety of sizes in multiple different locations within the photos library. You may get lucky and stumble upon it by using spotlight or manually digging around. 

Also look into repairing and rebuilding the photos library database, this may fix the problem for you automatically. Here is a source that discusses the process