It is really important to know that any early secular references to "christians" cannot be attributed to a particular cult or temple let alone one of the the Jesus cults. Early christianity was lumped with what was later perjoratively called paganism by the Romans since their beliefs overlapped as did the types of worship including prayer with arms extended. Early Christian graves for example are rarely distinguishable from pagans since they used the same symbols such as crucifixion, the shepherd carrying a lamb etc.
The same graffito as you show Fulltimestudent is presented in Gandy and Freke's The Jesus mysteries, Element, 2003. Here it is given a date between 193 to 235CE with the text suggesting a pagan initiate of the mysteries looks on at the crucifixion of a donkey headed man. "This represents his lower 'animal' nature, which he has put to death...so he may be spiritually resurrected".
I recall a horse-headed Roman god which was apparently mocked by contemporaries for its ludicrous looks, I wonder if this had any connection with the image in question?