its best if you know how to do it by yourself so i'll tell you how to do it.
first,open the image in photoshop.select the part that you want to crop by using the quick selection or magic wand tool. copy or cut it(whatever floats your boat). create a new document and select clipboard in the preset menu.select transparent in the background contents. then select ok. paste. save as .png file.
anyways, that's how i do it. photoshop is not limited on what other people can do, it is limited to what YOU can do.
i dont know if that will do. but at least i tried to help.