- Open your image in Preview (right-click, hover over Open With > click Preview).
- In the Menu Bar, hover over Tools > Adjust Size.
- In the Fit Into dropdown, choose 1920x1920.
- To avoid overwriting your original image, make a Duplicate file instead of saving now:
- In the Menu Bar, hover over File, choose Duplicate.
- A new window will pop up over the original one with a file called <filename> copy.
- Save this copy, then close Preview without saving changes to the original.