How do I assign the Main photo for my listing?

So i sell shirt with different sizes and colors. I create a listing and in the variations tab, i add 8 sizes and 11 colors. So in total 89 listings are created, 1 for parent and 88 for children. Whenever i search for my product on Amzon search, it pops up but the main image is some random color. How do i add a specific color image to that main image? Or a specific child variation perhaps?