Yes, the early church writings show that early believers used a symbol of a fish as an identifying mark. The Greek was Ichthus, and it stood for an acronym meaning, in Greek, "Jesus Christ, Son of God, Savior." The prohibition in Deuteronomy against making images of anything, whether it is a fish or otherwise, was not against making images, per se. Within the Mosaic Law itself, you have God commanding the Jews to make images. For example, for the Tabernacle, they were commanded by God to make two images of cherubs overshadowing the Mercy Seat. Moses was also told to make a brazen image of a serpent and put it on a pole. What this shows is that God did not negate the making of images, but only forbade the making of images for the purpose of bowing down to them. The early believers did not worship the fish symbol, they just used it as a sign to identify fellow believers so that did not violate any commandment.