Open the picture in Finder, Photos, Safari, or another app
Click the picture to select it
Press Command + C
Open the folder, document, email, or app where you want to paste it
Press Command + V
To copy from Finder, Control-click the file and choose Copy
To duplicate a picture file in Finder, select it and press Command + D
To copy a picture from a webpage, Control-click the image and choose Copy Image
To copy a picture from Photos, select it, then use Edit > Copy
To copy a picture to the clipboard from Preview, open the image and press Command + C