The pictures are from wall paintings at Doura Europos, a city destroyed by siege and preserved by time.
It is a safe bet that the subjects of the paintings did not live long afterward their images were laid down as the Sassanid Persians destroyed the City .

The Light horseman is pictured first and is in keeping with the general concept of Roman Cavarly or Gallic Auxiliaries up to the 2nd Century.

As to the Fully armored Cataphract (called by the Roman the 'clinibarus") his armor is relatively new in Roman use, as Infantry remained was the Arm of Decision. Nonetheless, after several negative encounters with the Parthian and Sassanid heavy horse, (despite the fact that the Romans often beat them), I am not surprised at the adaptation of the Persian Style Horse armor, although the "4 posted" saddle remains. (No stirrups yet.)
Tacitus tells us that this military saddle gave great stability and a point of leverage from which the mounted soldier man could use his weapons well.
Note the Helmet, which retains a bit of the flavor of the Imperial Gallic Helm, althought it appears riveted and not forged.
The Methods of the Fabrica (state owned arms factories) had begun to change.
It is rumored that the Emporer Julian may have stopped at the City during his expedition againt the Sassanid Persians.
The material I have does not go into great detail about the equppage of his cavalry, but perhaps the perhaps the best evidence of Cavalry Equipment is as shown.
JT