If you haven’t been paying attention, the newest thing in icons is font-icons.  That means creating vector-based images, putting them in a font, then assigning css classes to specific characters (icons) in the font.  It’s smooth, scalable, fast, and works on recent versions of all modern browsers.  There are already a few editors out there which you can use to create them, or just look on github for “fontello” to generate one.