Open the picture in Photos, Preview, Safari, Finder, or the app where it appears
Click the picture to select it
Press Command + C
Right-click the picture and choose Copy
In Finder, select the image file and press Command + C
To copy from a webpage, Control-click the image and choose Copy Image
To paste the copied picture, press Command + V
To copy and paste into another app, use Edit > Copy and Edit > Paste