Pictures is a folder not a file.
Pictures may contain photos if you manually store them there.
Photos is an application that stores its photo database inside the Pictures folder.
To get pictures into an email if you are using Apple's default Mail app, you can click on the photos icon at the top of the window and then on Photos from the drop down menu to open the Photos dialog. You can then drag a photo from there onto the email.


To open a photo form the Photos app in any other app, you can use the apps own Open option, normally in the file menu, and select the "Photos" option in the Media section of the finder window that opens.

Alternatively, you can if you want to use the Export option in the Photos App, to export a picture into a folder on your Mac and then access the photos from any App normally.