|Frida Kahlo age 12 (1919)|
You don't need permission to refer to Frida Kahlo in your film's dialogue. Free speech guarantees your right to reference a historical figure. More importantly, deceased figures cannot be defamed or have their privacy invaded (the two common basis for lawsuits involving the use of real people in fiction).
The painting. Mexican copyright law placed Kahlo's works in the public domain 25 years after her death, so you don't need authorization to include a photo of her painting in your film.
The Kahlo name. Certain rights associated with Frida Kahlo were retained (and disputed) by her family, most importantly, the right to use her name for commercial purposes -- for example, Mattel licensed the name for its Frida Kahlo Barbie doll. Although you're free to use Kahlo's artwork in your film, crafts artists should beware that use of the Kahlo name in connection with Kahlo-inspired works sometimes can cause a problem.