Open the text editor, e.g. Notepad or Wordpad. Any word processing program will do in fact. Load the template file you wish to work on.
Images require the file name of the image including the proper file extension.
Click on File, then Save. Save the file back to its original directory which should be the CCAtlas directory or a sub-directory within it, depending on where you got the file to work on from. The original file name will be retained when you try to save. Do not make any changes in the name or extensions or links in other files will no longer work. Open your Web browser and load the file you just saved.