"...had never traveled" is past perfect tense. It means he did not travel outside of France before painting the jungle scenes. "...he never traveled..." would be past tense. This would mean that to this day (or since this is someone deceased, for his entire life) he never traveled outside France. In this particular case the meanings are similar but only because he painted these paintings "in the final six years of his life."
Imagine instead if he had painted those paintings in his 30s and then lived another 30 years. "He had never traveled outside France" would indicate that he didn't travel outside France before painting them, but leave open the possibility that he may have traveled outside France later in life.