I was reading on the NASA website about FITS and the text states that FITS is not just an image format.
On this page they write:
FITS was originally developed in the late 1970's as an archive and interchange format for astronomical data files. In the past decade FITS has also come into wide use as an on-line file format that can be directly read and written by data analysis software. FITS is much more than just another image format (such as JPG or GIF) and is primarily designed to store scientific data sets consisting of multidimensional arrays and 2-dimensional tables containing rows and columns of data.