Greenfoot does not work with the spritesheets as a whole. You have to split the sheet into its individual images for Greenfoot to use them. Therefore, it is up to you whether or not to use a spritesheet or just give names to your images the represent the order of use (for example: 'image1.png', 'image2.png', 'image3.png', etc.) that can be easily retrieved by building the string name ("image"+imageNumber+".png"). As an alternative, create an array of the images and use the array's index to get the appropriate image from the imageNumber value. Another alternative, is using the GIFActor super(support)class that is available on the site.