The Greek word for Fish, (Ichthos) are also the first letters of the words, “Jesus Christ God and Savior” Some people Translate the letters to mean “Jesus Christ, God’s Son, Savior” It along with anchors for hope, and a sign of the cross, (The cross was not used early on as it was a symbol of very bad criminals.) Gardens, feasts etc were used to show the great teachings or mysteries of the Church.
I’ll have to re-read, I know that when the Church was made legal, and given basilcias, the statues that had been in the buildings were renamed, and became representations of Christ, our Lady and Saints. I don’t know about prior to the freedom of the Church.
There are any number of graffiti at the graves of Saints, from the first second and third centuries asking for their prayers. As well as some frescos (wall paintings) depicting our Lady with the infant Jesus.