Open the picture in Finder, Photos, Preview, or a browser
Click the picture to select it
Press Command + C
Open the destination folder, app, or document
Press Command + V
To copy from Finder, right-click the picture and choose Copy
To copy from Photos, use Edit > Copy
To copy from Preview, use Edit > Copy
To copy from a webpage, right-click the image and choose Copy Image
To duplicate a picture file in Finder, select it and press Command + D