JIMI

I have been attempting to obtain pixel data from images using Jimi. Specifically, I used Jimi.getRasterImage(String fileName) to obtain a JimiRasterImage object. Once I have the JimiRasterImage object, I have been extracting the pixel data. I have had no problems with JPEGs, but when I attempt to use a GIF or a TIFF(uncompressed) I am getting bad values for the pixel data.
Any advice would be greatly appreciated.
Tim