Assume that you want to create a toolbar like the Mail application. The Icon Design Guidelines in the Apple’s HIG says that the icon should measure no more than 19 x 19 pixels and should be in the PDF format. However, PNG works fine, too. More importantly, if we want to have the inner shadow effect,
than the name of the image must end with “Template.” As far as I can see, this is not documented in the HIG.