Where are the photos in Contacts stored on my MacBook (High Sierra)?
First use Finder and Go > Go to Folder... > (enter the following path)

~/Library/Application Support/AddressBook/Sources/


From there navigate to the only subfolder (it should be a string of numbers and letters) and lastly, enter a folder called 'Images'. You'll see your contact image files in .jpeg format in this folder.

For reference the end path should be:

~/Library/Application Support/AddressBook/Sources/#######-###-###-########/Images/


I hope this helped!