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