For what it’s worth, the Trivia section for this movie at IMDB.com says “In the final shot we see the Ark passengers disembarking with an obvious painted background depicting Zyra. This bad painting was tacked on for the film’s sneak previews. Originally, producer George Pal wanted to depict Zyra as a miniature set, but Paramount shipped the film out before this could be done.”
I also remember reading somewhere that they just ran out of money at the end of production and that painting was all they could afford. (Sorry, no cite.)