Where are the photos I imported into Photos on my Mac?
When you first use Photos, you create a new library. New photos and items that you add to Photos are stored in this library. The library appears in the Pictures folder on your Mac.
The library automatically becomes your System Photo Library, which Photos accesses each time you open the app. Only your System Photo Library can send or receive photos transferred by iCloud Photos.
You can also create additional photo libraries on your Mac and on other storage devices. You can use as many different libraries to hold photos as you like. You should only use the Photos app to access the photos in a Photos library.
WARNING: To avoid accidentally deleting or corrupting a Photos library, do not manually access or alter the contents of a library in the Finder. If you want to copy, move or transfer files, first export them from the Photos library.