Interpretations of the Alexamenos graffito

Blasphemy
The crucified man with donkey head has been seen as mockery of worship of Christ on the cross. From this the name graffito blasphemo given to the drawing by Harold Bailey (1920).

Wikipedia tells

The inscription is accepted by authoritative sources, such as the Catholic Encyclopedia, to be a mocking depiction of a Christian in the act of worship. The donkey's head and crucifixion would both have been considered insulting depictions by contemporary Roman society. Crucifixion continued to be used as an execution method for the worst criminals until its abolition by the emperor Constantine in the 4th century, and the impact of seeing a figure on a cross is comparable to the impact today of portraying a man with a hangman's noose around his neck or seated in an electric chair.
wikipedia 

If this interpretation is true the Alexamenos graffito underlines the importance of crucified Jesus in early Christianity. Since the drawing - if dated to 2nd or 3rd century - is hardly inspired by an actual crucifix in a place of worship it may be inspired by frequent references to the Crucified by Christians.


Onolatry 
Actual donkey worship has also been suggested as explanation although the picture depicts a man with donkey head on cross.

It seems to have been commonly believed at the time that Christians practiced onolatry (donkey-worship). That was based on the misconception that Jews worshipped a god in the form of a donkey, a prejudice of unclear origin.

Tertullian, writing in the late 2nd or early 3rd century, reports that Christians, along with Jews, were accused of worshipping such a deity. He also mentions an apostate Jew who carried around Carthage a caricature of a Christian with ass's ears and hooves, labeled Deus Christianorum Onocoetes ("the God of the Christians begotten of an ass").
wikipedia

IMHO this might provide additional background to the enigmatic crucified as a rather total mix of prejudices and misinformation about Christianity among the people of Rome during the period it was a forbidden religion.


Other suggestions

Others have suggested that

• the graffito depicts worship of the Egyptian gods Anubis or Seth, 
• or that the young man is actually engaged in a gnostic ceremony involving a horse-headed figure and that rather than a Greek upsilon it is a tau cross at the top right of the crucified figure.
• It has also been suggested that both the graffito and the roughly contemporary gems with Crucifixion images are related to heretical groups outside the Church.

wikipedia

Anubis or Seth are indeed often depicted in art as human body with animal head in Egyptian art. At least in Egypt there seems to be nothing to link these ancient gods to cross.

Many things can be said under the shady concept of gnostisism.