Given that it's a flattened image with no alpha mask whatsoever, there's no "transparency" to select.

If it were layered, and one of the layers semi-transparent, then there you go, but it's flat.

I was thinking the same thing as spedler - I'd just recreate it given how simple it is, and make the grey box semi-transparent so you can see what's underneath.

I know you were looking for something better as far as instructions or help, sorry. Good luck-
-Lew ;-)