You want to reduce the size of photos with Apple Mail before Sending, you don't need the circuitous route via an image editing program to go. Apple's Mail Client has the necessary Feature is already on Board.
Photos with Apple Mail out in front of the Ship
With Apple Mail, you reduce the file size of photo attachments to your E-Mails in the blink of an eye.
You create with Apple Mail, a new E-Mail by clicking in the menu "file" click "New E-Mail", or alternatively the key combination [command] + [N] press.
Locate the photo you want to send, in the Finder, and drag it via Drag&Drop into your newly created E-Mail.
Select in the drop-down menu on the right bottom of your E-Mail one of the options of "original size", "Large", "medium" or "Small" from.
Apple Mail displays on the left bottom of the message, the new E-Mail size.