Who was the first person who ever painted?

Determining the age of paintings and locating more examples of very ancient paintings have both become more possible in recent years. As new discoveries and scientific techniques develop, we will probably keep pushing back the "earliest" dates.
Recent discoveries in Indonesia and Spain have done a lot to expand our knowledge of early paintings.
In 2014 on the island of Sulawesi in Indonesia, cave paintings were found dating to more than 39,000 years ago making them almost as old as what were thought to be the earliest paintings previously found in Spain. The Indonesian paintings include "spray-painted" outlines of hands and a drawing of a deer.
In early 2018, cave paintings were found in Spain that are believed to be about 65,000 years old. This period is associated with Neanderthals, or homo Neanderthalis, who were predecessors of our modern humans, homo sapiens sapiens. One shape depicted resembles a ladder.
https://www.nature.com/articles/d41586-018-02357-8?error=cookies_not_supported&code=6093b09f-a449-41c4-8ad6-de9a34c7f2ca

https://www.nytimes.com/2014/10/09/science/ancient-indonesian-find-may-rival-oldest-known-cave-art.html

We'll never know the answer to that question, because the first painter existed long before written history.
In 2012, a discovery was made in caves in Spain of paintings showing a red sphere, handprints, and images believed to be seals. Testing showed one of them to be at least 40,800 years old. Some scientists posit they were made not by early humans, but by Neanderthals. These are considered the oldest known painting to date.
The famous cave paintings in Lascaux, France, date from the Palaeolithic era and are believed to be as much as 20,000 years old. They depict mostly animals, probably ones that were being hunted by the people who painted them, alongside some human figures and mysterious "signs."
Who discovered it was possible to make pigments to color their drawings with natural minerals? Why were these drawings done—was it to document deeds accomplished, or did they have some kind of spiritual significance to the painters? As of now, we can only guess.

The El Castillo cave images may well not be the oldest examples of painting. It's entirely possible another cave or cache may be discovered someday that contains even older examples of what appears to be a basic human trait: creativity and the urge to express it.