In Photoshop click File > New > Blank File. In the "New" window, size it to your requirements, set color to RGB and background to transparent and give it a name if you wish. Open the file containing the button graphic you want to use. Using the Magic Wand tool, select the part of the graphic you want to use, size it, then Copy and Paste the selection onto the the new item, choosing Paste as New Layer. Save your image in .png format.