Tell me where it is because I've looked for it. Someone didn't believe me once. I know he flipped the finger to the "young upstarts" who took over country music and left him in the dust. I notice he's joined the neocountry artists but we have fans here so I won't belabor the point.
A google IMAGE search of the words "Johnny Cash" and "finger" will get you to a couple of sources.
I think it was originally from the early 1970s. Later in the 1990s, American Recordings used it to "thank" Nashville for the lack of airplay, recognition, etc.
There is also a photo of Jerry Lee Lewis letting both birds fly. It was a posed shot so I don't know the story behind it.