Joe Workman did a series of videos on Foundation. This one explains the grid system. It’s worth watching. You can find more videos on his YouTube channel and additional documentation at docs.joeworkman.net.
If using the 1-column stack only as a container for the 2-column stack, I recommend its removal. As described, it’s without a purpose.
Your browser is an infinite width (as wide as your monitor will allow). Your site is a fixed width, be that 900, 1000, 1100 pixels, or other. Columns keep content inside the site width. When you resize your browser so that it exceeds the site width, you will always have black on the outside of your content. I don’t recommend putting that image outside of a column though. If someone looks at your site on a 5K iMac, then your picture will be several thousand pixels away from the right-most edge of your site.
The second thing at play is the gutter. When you place anything in a column, there’s some margin. This is because you don’t want content at the physical edge of your browser. It looks bad.
If you want your picture to touch the browser edge/the full site width, you can put the image in a 2-column stack. Then, in the column settings, under Row Setup, set Gutter to Never. In the left of 2 columns, place a 1-column stack. Put your text in that. It’ll provide the gutter for your text. Next without a gutter should be avoided.