Windows 7 taskbar is one of the best features of the OS, with pinning and jumplist, the taskbar is really awesome. Here is a nice freeware app, W7 Superbar IconChanger, which allows users to change the Windows 7 taskbar icons for any applications. When you run the application, it lists out all the open apps in the taskbar and you can manually change the taskbar icons. Generally all apps have its own icons for taskbar by default.
You can select the application for which you want to change the taskbar icon. The app shows the current app icon and also has a button to change the icon. You need to have an icon file to replace it.
The changes will be only visible, if you close the program. You will be asked, if you want to restart the explorer. So please save your work before you change the icons.
Related: You can also check out 7Conifier to change the taskbar icons in Windows 7.