RTFM.
- (void)setApplicationIconImage:(NSImage *)anImage
Sets the receiver's icon to anImage. This method updates the dock
application tile. anImage will be scaled as necessary to fit the tile.
The following code sample shows how to get the application's icon image
and then restore the dock's image:
myImage = [NSImage imageNamed: @"NSApplicationIcon"];
[NSApp setApplicationIconImage: myImage];
Source:
Cocoadev