“Alexamenos worships his god.”
The fact that he was crucified was still an insult, up to probably 200 CE ish at least. Crucification was an especially degrading death - not just about killing you, but making sure it was long, drawn out and humiliating. (Longinus would have been doing something kind. Think about giving someone vinegar on a sponge to drink - to keep them alive just a little bit longer is drawing out the torture. You wanna be stabbed with the spear.)
Early Christians used the fish as their symbol for Christ. If the resurrection were historical (sorry NT Wright - I don’t think the tomb was empty) - a fish would make more sense for accessory choice.