Image border in Preview app
You can toggle "Show Image Background" in "View" menu (⌥ alt+⌘ cmd+B). This will produce "chess" pattern instead of the clear color. Example (I've set white color around this rectangle to be transparent):
You can toggle "Show Image Background" in "View" menu (⌥ alt+⌘ cmd+B). This will produce "chess" pattern instead of the clear color. Example (I've set white color around this rectangle to be transparent):