All FITS images require ds9, Aladin, or AstroImageJ to view, although they may load into other software with loss of dynamic range.
Unprocessed FITS images have not been corrected for the "dark" signal that is present even without an exposure, or for the variation in the response of the telescope across its field of view. To use this data requires special software (Aladin and AstroImageJ, for example), an understanding of image processing, and some patience.
Processed FITS images have all of the science information, often including celestial coordinates. There is one image for each filter. They appear only in grayscale, and must be combined to make visually colorful images, but viewed in ds9, ImageJ, AstroImageJ, or Aladin they reveal a wealth of detail.
Panoramic, zoomable, on-line images in color or grayscale show the full dynamic range and are visually informative, but at this time they cannot provide quantitative information.
Smaller compressed JPG or PNG images, used for documents, email, websites, and paper prints, are often stunning, but also not usually good for quantitative analysis.
Keep in mind that while the FITS image files are very large (128 MB, typically), the panoramic on-line images are viewable with low bandwidth requirements on a desktop, tablet, or cell phone. For a printable image, or one to use in a report or document, request a smaller downloadable JPG or PNG file. If you are doing research that requires a measurement of magnitude or position in celestial coordinates, request the processed FITS images.