How were the coins actually inscribed?

Tosfos, Meiri, Rashba: There were not images of an old man and woman on one side and a young man and woman on the other, as it is prohibited to make the image of a person. Rather, the words 'old man' and 'old woman' were stamped on one side etc. Rashi also explained this way concerning the coin of Dovid and Shlomo.

Ein Eliyahu: Tosfos write elsewhere (Avoda Zara 43) that a partial image is fine. If so, they perhaps merely inscribed images of their heads on the coins.

What did the form of the coin of Avraham represent?

Rashi: The young man and woman represented Yitzchak and Rivkah.

Maharsha: It marked the miracle of Avraham and Sarah themselves physically returning to their youth, with the pregnancy and birth of Yitzchak.

Yalkut ha'Gershoni: It provided a message to those who are still young that they should anticipate their old age and begin to increase their charity and good deeds.

Ben Yehoyada: It showed that since Avraham and Sarah loved each other for their righteousness, this did not decrease with time, and it was as strong as when they were young.