Using the Layer Styles dialog you can remove any white pixels by changing the Blend If slider. Drag the white triangles to the left. Any white background will now become transparent. In order for you to access this dialog the layer can not be a "Background" layer. If it is, click the Padlock Icon to unlock it. Also there must not be any layers beneath. Save the file as a PNG 16 and check the box to include transparency. Note: While the file will have transparency, the Finder or (presumably) Windows Explorer will show the transparency as white.
... View more