First convert it into a .ico file. you can do that using this [link] Then right click the Photoshop icon and create a shortcut, then right click the shortcut and select properties and then select the shortcut tab. There you should see a button that says "change icon", click it and find the icon you want then click ok. Done
Then right click the Photoshop icon and create a shortcut, then right click the shortcut and select properties and then select the shortcut tab. There you should see a button that says "change icon", click it and find the icon you want then click ok. Done