You're probably heard it pronounced both ways and after all these years of double pronunciations it really doesn't matter but here are a few little tidbits about those .gif files. Gif stands for Graphics Interface Format as they are popular computer graphics due mainly to three reasons...they are small in size ,they allow for a transparent background and they can be animated! The gif file format is owned by Compuserve.
Now for the correct pronunciation ...the Oxford Dictionary says both ways are correct. Steve Wilhite the man who actually invented the format for Compuserve inventory however...JUST LOVES PEANUT BUTTER SANDWICHES! So for the purist the answer is JIF.