Explanation: Roman cities had significant literate populations, and so those few preserved remnants of buildings frequented by private citizens, such as the buried city of Pompeii, are often covered with graffiti, text and image alike, of varying themes.
One such recurring theme is drawings of dicks. Normal dicks, big dicks, little dicks, dicks with wings... the penis was considered a symbol of fertility (both kinds) and good luck, so rather than being inappropriate, it was funny and generally complimentary! (Though the sheer amount of graffiti certainly bothered some - as one Roman graffito ironically noted, "O walls, you have held up so much tedious graffiti that I am amazed that you have not already collapsed in ruin.")
Maybe don't draw it over your pal's newly painted fresco though.