Now it came to pass when he drew closer to the camp and saw the calf and the dances, that Moses' anger was kindled, and he flung the tablets from his hands, shattering them at the foot of the mountain. [Ki Tisa 32:19]
According to the Talmud, the Tablets were each 6 x 6 tefachim in size, and together they filled the Ark, leaving no space [Bava Basra 14a]. From this we see that the Tablets were square in shape.
Furthermore, there is a halachic principle that the vessels of the Temple are only valid if they are "intact and full". Thus, it was a legal imperative that the Tablets filled the Ark completely leaving no space. Obviously, ths would preclude them from being rounded in shape.
The concept of tablets with rounded tops is actually non-Jewish in origin, being derived from Roman tradition (the nation that destroyed our holy Temple). Nevertheless, the image found its way into our books due to the non-Jewish censorship of printing spanning many centuries. As a result, today there are even religious Jews who depict the tablets as being round on the top, contrary to the Talmud.
It is a mitzvah to publicize at every opportunity that, according to Jewish sources, the Tablets are square in shape.
[Based on Sichas Shabbos Parshas Ki Tisa 5741 - Lubavitcher Rebbe]