Although I don't have Photoshop, you could try using a magic wand tool (usually a wand type icon) and select the black area, and then go to a menu and "invert" the selection, which would get just the button itself (not just the square with the image in it)
Keith
ps. I don't know anything about photoshop, but I imagine it has this as even old software programs had magic wand tools, and an option to invert a selection.