Open the photo in Finder, Photos, or Preview
Select the photo
Press Command + C
Go to the destination folder or app
Press Command + V
To duplicate in Finder, select the photo and press Command + D
To copy from Photos, drag the photo while holding Option
To copy from Preview, choose Edit > Copy
To copy from a webpage, right-click the image and choose Copy Image
To copy multiple photos, select them all before pressing Command + C