1) Download & click on unarchiver.
2) Open up the folder and copy the icon you want to use.
3) Paste the icon in the 'Icons' folder in the theme you are using.
4) Have 2 copies of the icon.
Name it 'Instagram.png' & 'Instagram@2x.png'.
5) Respring your phone or activate theme in winterboard.
Things to take note:
- While renaming the icons, you'll have to follow the exact app name (capital letters and spaces) so that it will work.
Refer HERE for screenshots.