A fits image file is capable of storing 64bit data which gives a huge dynamic range to images, although you'll be lucky to find hardware that can supply better than a 16bit image. Regardless, this means it can store very bright and very dim objects in the same file. It is not meant as an image transport system like a BMP or JPEG, but as a data storage medium. If you can save images to fits in the first place then do so and convert to something else from there using the fits as the master.
Nasa has a list of viewers and convertors here :-
http://fits.gsfc.nasa.gov/fits_viewer.html