A possible answer: Professor Yaakov Shpiegel wrote an entire essay on imagery in Jewish cemeteries. I'll translate some of his points:
"The archaeological findings from the time of the Second Temple show, that in the architecture and the art of decoration of Jewish graves there was a strong Hellenistic influence, but even so, it was an entirely non-iconic art. On the faces of decorated graves and the hundreds of caskets and sarcophaguses that were discovered until now, there is not to be found even a single motif taken from the animal kingdom, only examples from the plant kingdom and geometrical shapes..."
I'm not familiar with pre-Hellenistic Jewish graves, but if there aren't images on such graves, then it seems that the origins of the decorations came from Hellenism. However, in those early days, the image subjects were kept in the boundaries of Halacha.
"At the turn of the 2nd century and mostly in the 3rd century CE, a complete turnaround happened, and a new leaf in the birth of Jewish art was turned...it seems, therefore, that the images in the Jewish catacombs in Rome, that in their time caused tremendous controversy because of the multitude of idolatrous subjects within them, were not a singular, passing phenomenon, but the birth of of a general evolution, that was rooted in in the influence of the general non-Jewish population of the area, on one hand, and in a relaxation of the Torah prohibition on the other hand.
In one [burial] cave there are buried rabbis and their families in dug graves and simple caskets, and next to them are buried other people (whose identities we do not know) in caskets decorated with images of humans and animals. Rabbis considered 'holy', who in their lifetime abstained from looking at a coin that had an image of a person, were buried in simple, non-decorated caskets next to Jews who were buried in caskets decorated with idolatrous-mythological imagery. It may be assumed that those buried in the pagan caskets were influenced greatly by the strange culture - a kind of 'advanced' Jews, and maybe even Hellenistic Jews. Even so, it seems that these were good Jews, for if not, they would not have requested to be carried from far-away to be buried in the cemetery where Rabbi Yehudah Hanasi was buried...a few researchers attempted to give an explanation to the phenomenon in different manners. There are those that attempted to explain that this bears witness to the existence of several cults in the nation whose faith was different from that of the sages, and therefore developed for themselves a system of symbols, that though was taken from the idolatrous world around them, but they gave those symbols different meanings...others attempted to explain the phenomenon with accordance to the acceptable Halacha, in light of the history of the relations between Jews and non-Jews in the land. To say, that the sages found here a place to be lenient for a variety of reasons..."
"After this period, we do not find images of people or animals on tombs, more so, we do not even find any sort of decorations on the tombs, and the only thing on the tomb is the inscription. Later on, near the end of the 16th century, decorations and different images began to appear upon the tombs...in these cemeteries there are those that are carved with Tanachic imagery that are connected to the name of the deceased...I could not find rabbinical references to the phenomenon, and it appears, that the question never arose as an issue. As much as I have been able to find out, the first question on the matter was written only in the 19th century...it may also be said, that not everyone saw in this a Halachic question, because the cemetery was not considered in their eyes like a synagogue and a place of holiness, and therefore there was no need to be strict like in the synagogue.
"As we wrote above, the statues and the images on the tombs were related mainly to the name of the deceased. Although we have found those that are related to his profession...Rabbi Yosef [ben Naim] expanded on the idea, and in another place wrote:
"It seems to me that it is needed to clarify on this...it shows that the deceased enjoyed his work...happy he was in this world and he will be in the next world..."