I just assumed it was an automatic thing, depending on the thumbs/nuts awarded, but have never really paid attention to how often it changes up. I would think every day, but could be wrong. Strangely, I tried to find Frenchy's photo, and ended up going to the database entry to even see what it looked like. So...?? Who knows.
P.S. It IS a lovely photo, which I didn't see when it was featured. Glad I took the time to go look for it.