That's because you've named the images similarly.
Your new image is "morning.jpg" and the extra image is "morning_haze.jpg".
Zen Cart allows you to set multiple images for products, and uses the product's image name as the "base" for finding all "like" images for the product. And, then displays any it finds in the "additional images" section below the product details, as you described.
This allows some products to actually share some images, depending on their filenames (sometimes a product is a derivative of another product, and the additional images are useful that way, meaning less image files to prepare and upload to the server), and the naming convention allows for simple setup of additional images.
You have a few options:
a) rename your new product's base image filename, both the file and the image name in the product details in your admin
b) turn off the entire additional-image support in your admin settings
c) if the extra image isn't actually used by another product, you could just delete it off the server, and it won't show up anymore.
Related FAQ articles:
https://www.zen-cart.com/tutorials/index.php?article=315
https://www.zen-cart.com/tutorials/index.php?article=58