Paulus,
I prefer "rock slab" to "rock panel". Being a rock man, in terms of huge crags, the term "slab" seems most appropriate to me. Brings a tingle to the spine, "slab" does. "Slab" in rock climbing terms usually means a rock face between 30 and 75 degrees. Perhaps a bit irrelevant in terms of rock art, but slabs in rock climbing terms tend to be big, clean, and obvious, in fact perfect for rock art.
TE.