From a short but very interesting post by Larry Hurtado:
The earliest of the various “christograms” is the tau-rho (the capital rho superimposed on the capital tau, the device resembling a capital “P” superimposed on a capital “T”). We have examples of this device in several copies of NT writings dated to ca. 175-250 CE (P75, P66 and P45). And it’s still more intriguing that the letters in this device (also appropriated from prior non-Christian usage) don’t represent any name or word. Instead, the device is used as part of the special way that the Greek words “cross” and “crucify” are written in these manuscripts, and it seems intended to serve as a “pictographic” representation of a crucified figure, Jesus. This makes it the earliest visual reference to the crucified Jesus, some 150 or more years earlier than what art historians have tended to see as the first depiction of the crucified Jesus.