WHAT NOW I'M DOING TWO POSTS ON ONE DAY!!
I was inspired to do this after posting my GIF just minutes before this one. There's been controversy for years about how to pronounce the term, and here's definite evidence:
CompuServe used to distribute a graphics display program called CompuShow. In the documentation for version 8.33 in the FAQ section, it states:
The GIF (Graphics Interchange Format), pronounced "JIF", was designed by CompuServe and the official specification released in June of 1987.
So there you have it.
Peace out; I'm still not a brony, and you can't convert me.