Were angels depicted in early Christian art? No. Not really. Early Christian art did not represent angels very often, and when they did appear in frescos, angels were often represented as other things. For example, in some paintings, an angel was indicated by doves. In another painting, an angel or messenger, is depicted as "the hand of God."
Roman Emperor Constantine changed things when he formed Christianity into a State religion through his council of Nicea in 325 C.E. It would be after this that angels would eventually be depicted in paintings and art with wings and halos.
The earliest known angel image depicted in Christian art is a fresco that dates back to the second century. It is the annunciation scene found in St. Priscilla cemetery. The same subject matter was executed again a century later, also being found in the St. Peter and St. Marcellinus cemetery. Both paintings include the legendary archangel Gabriel, dressed as a man in a robe.
A fourth century painting entitled, The Good Angel, also shows an angel that has a human form. These angels, we should note, did not have any wings or halos. Angels with halos and wings were simply not depicted until after Constantine.
It seemed that before this, angels in paintings were purposely strayed away from, perhaps so as to not mimic other pagan deities' depictions. In fact, classical winged gods, such as Cupid, Hermes and Perseus, as well as the ancient winged gods of Mesopotamia were likely the inspiration for wings that would eventually be added to angels after the fourth century.
Halos did not float above the heads of the few angels depicted in early Christian art until after the fourth century, as well. A halo was a symbol of character, traditionally only painted above pagan gods or Roman Emperors. It is likely for this reason that halos above angels were avoided. When Christian Emperors began having their portraits painted with halos above their heads, halos were soon the rage finding their way to adorning the head of Jesus, then later the heads of angels by the fifth century in Christian art.
Of course, since the fourth century, angels have found their way into thousands of Christian art pieces and paintings. Many of them show angels with wings and halos, but these were the creation of artists and not how angels truly appear.