What is the Default Location of Photos in Photo application? I see many photos in Photos application. 
However, I cannot add them to my mails without knowing their exact location.
I am using El Capitan.
Of course, I can take a screenshot of originals always but I see this as a way of duplicating things, losing accuracy and causing trash. 

I would like to get the location for the internet browser like Google Chrome, Firefox, ...

You cannot drag and drop directly from Photos to the SE's picture uploader neither. 



How can you add a photo in Photos application to a web email window?
Right click the photo > Share > Mail



Alternatively, from within Mail itself
Window menu > Photo Browser