Select coulour range. Choose pure white by clicking on the foreground icon.
You should have the marching ants around the excessive white in the image.
Dont worry if parts of your image are also selected.
Select inverse and crop. Then deselect.

Stop your recording action and voila...

The advantage of using the square format for your signature is that you can
use this action for landscape, portrait and square images.

And remember: There are many roads that lead to McDonalds. This is just one of
many ways of achieving the same result. This way just happens to work for me as
I work a lot with actions and scripts. Mave fun.