If you need to make a Photoshop document of your application icons complete with the transparent background there is a quick way to create them.
1. First, CTRL-Click the application icon and chose Show Package Contents from the contextual menu.
2. Browse to the Contents/Resources directory and find the application icon file with the .icns extension.
3. Double click the icon file to open it in Preview app and save the file to your Desktop as PSD format.
4. Now you can drag the PSD file to Photoshop and you will notice that the icon is there, but the alpha channel is not applied, so you don’t have a semi-transparent background with your icon. Fortunately if you check the Channels tab you will see that the alpha channel is indeed saved with the icon. Apple-Click (Cmd-Click) the alpha channel icon to make a selection out of it.
5. Final step is to apply the selection to your icon. First you need to make a layer out of your background. Just double click the icon to make it into a layer and then with the selection from the alpha channel still active click the mask icon on – third small icon from the left in the bottom of the layers palette.