Grass_Image_MosaicService - This service could be used to mosaic 2, 3 or 4 images and extends colormap to range of all images.
Operations:
mosaic - Mosaics up to 4 adjacent images. A new image will be created with extended colormap. The first image will be on top of image stack. Image colors are always going to be different on each tile (each tile was taken at different times, with different lighting, different terrain, etc.).
