File editor help

[holyplankton]

I recently installed the Gods and Kings pack (I really want to play the Game of Thrones mod) and since I did I've been getting a missing texture message. While it's not causing the game to crash or work incorrectly, it is a constant annoyance and I have to close the error message several times every time I start a new turn and the Babylonian ruler picture in the game setup screen is just a red box.

I believe I've found the way to fix the problem, but it requires me editing the code for a file and I can't do it with notepad. I was wondering what editor program I should use to fix this issue. Any help would be greatly appreciated.