I don’t like the default icon for Preview.app in OS X.
The sight of that child, giving me this creepy grin, sitting here in the Dock while I’m browsing pictures, is just weird.
There are two ways to make him go away.
The hard way
I call this method the hard way because it involves having another .icns file (the filetype for OS X icons), and navigating through folders supposed to be hidden. But it doesn’t matter, you’re a hardcore user!
Right-click Preview.app and click “Show Package Contents”.
Navigate to Contents/Resources.
Replace Preview.icns with the icon of your choice.
The easy way
This method allows you to replace an icon with another application’s icon.
Enter Cmd+c to copy it.
Another advantage of this second method is that it also works for disks, folders, and about any file.