Sorry for the late response. In real world it's part of a Sufi Muslim belief where you take on something impossible or almost impossible to show how feeble and useless the real world. As far as I know almost all the tortoise trainers are part of such Sufi branches and believers. They did this as part of their beliefs. It's almost impossible to teach anything to a tortoise in real life. If you search about the painting you'll see that painter had 2 versions of this and later one had 6 tortoises showing that instead of solving his issues or problems or trials he add more on. Also the painter Osman Hamdi Bey was part of Sufi Muslim believer. It's said that he painted himself as the tortoise trainer. I hope this cleared somethings.