Recover photos from thumbnails in Photos app
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