The most common reason for using something like Grayscale or Sepia on a still image is because you have addressed the rest of your Windows Movie Maker project with the same video effect. In this way you are altering the entire film to reflect this video effect. This is not always necessary on the still images, depending on their context.
However, this usually works better in reverse, when the video clips are in color and the still images are altered with video effects like Grayscale or Sepia. What is important to remember is that the still images, if they are not altered with any kind of photo motion, will feel removed from the normal flow of your video. In this way you can actually use a different color pattern to accentuate its removal, and this can reflect your choice to either use or avoid the color affects. Do not feel as though it is entirely necessary to use the color effects on the still images at all times in your Windows Movie Maker projects.