The praying figures with outstretched hands like those depicted in ceiling painting in the catacomb of saints peter and marcellinus are called:

Orans (or orante in the Latin original).

A believer who prays with outstretched hands is a very ancient religious representation. Such figures were found in pre-Christian cultures, such as Sumerian (3.000 BC). Jews also practiced this kind of worshipping, and early Christians probably adopted it from them, as it resembled of crucified Christ.