Change star rating images
You can find these images in the following directory Website\images\ratings. Replacing these images with new ones will change the rating images across the store.
But if you want to use some different format for the images for example if you want to use the png format for the new version of the images. For this first you have to create and put these newly created images into Website\images\ratings folder and then edit the App_Code/NavigationHelper.cs file's
public static string GetRatingImage(LSDecimal rating)
method and change the extension in following code
string ratingImage = page.ResolveUrl("~/images/ratings/rate_" + tempRating.ToString() + ".gif");