Open the image in your browser, app, or preview window
Right-click or Control-click the image
Select Save Image As or Export
Choose the folder where you want to save it
Enter a file name if needed
Select the file format if prompted
Click Save
If using Preview, go to File and choose Export
If using Photos, select File and choose Export
If using a screenshot, press Shift + Command + 4 or Shift + Command + 3 to capture and save it automatically