Where are Photos individual images stored

As Matti mentioned your best bet is to export the originals out of the libraries

with the following settings:

Each image file will have a corresponding XMP file which will contain any metadata (title, description, keyword, etc.) that you added to that image in Photos. If you import them into another Photos library at some later date the metadata will be added to the library.

Each group of photos will be in a folder with the name and date of the moment they are associated with. That will give you some help in identifying them via the Finder.

The safest way is to export as originals (notice that it does not include any updates to the dates, locations etc you have made to the images). That should not take too long.

Or ctrl-click to drill into the photos library package and search where the original imported (not referenced!) images might be and copy (or move if you don't need that library anymore!) the images out.

, have images from 2012 and 2013 but no folders showing images from other years,

In the library package the originals are in the Masters folder. The subfolders of Masters are labelled by the date of the import session (year > month > day > time), not by the date the photos have been taken. So, if you created the library in 2013, all photos you imported at that time will be in the folder 2013, also the older photos. And the yearly folders may change, if the library is repaired or upgraded. Have you found all your Photo Libraries? Looked on all external volumes you have regularly connected to your Mac?

Why not? The imported images should be there in some subfolder. Unless it was a referenced library package.