I wanted to capture the iTunes icon for a related blog post. I couldn’t remember how I extracted icons in the past, either or small app or a screen clipping. I figured there had to be an easier way and sure enough there was. I found it at Keith-Knipling.com.
The short version is to "Show Package Contents" for the application in finder. Then browse to the resource folder in the package contents and locate the icon(s) you want. The files typically have a .icns extension. Open the file in Preview and do a File -> Save As to save it as the file type and name you want. Be sure to save it to a different location.