Adding blank space around an image, by resizing the zone of a picture without changing the size of the image itself in MacOS Preview app, is not possible natively. However there is a workaround:
- Open the image in Preview
- Select all the content with
Cmd+Aand copy it withCmd+C - While all is still selected, delete the image with
Del - Resize the zone to the desired dimensions in Tools > Adjust Size
- Paste the original image with
Cmd+Vand reposition it if necessary - Save the file