Thanks To use a dock icon first you need to have a dock software such as famous ObjectDock for example. I reccomend you do a google image search "objectdock" just to see what it essentially is. Then you would right click on some of your icons in ObjectDock and go to Dock entry properties. And from that menu you can easily change the icon to whatever image that you have on your pc anywhere on your hard drive.
If you want to use it as a desktop icon and not dock icon, then you would have to convert it to .ico first. For that there are many free converters online an even a free photoshop plugin for .ico support. Just google it.