There is no add button or link on Published screenshot page. It's only show "How to add screenshot https://www.mql5.com/en/articles/999#para4".
But I dont know How do I add screenshots to my market product?
You add the screenshots when you create the market product.
Note that the one you upload last will be the first visible screenshot so you have to upload them in reverse order, and then need to be 640x480 resolution.