I want the picture to expand the length of the view port.
Those are margins of one of your elements. Change background colors and you will know which element it is.
My bad, background colors will not help to find margin of this element.
Better idea is to change margins on a few elements and see what is working or use developer tools, as Annestezia below me suggested.
Do you use browser dev tools? If not, try to: mdn Debugging CSS, you’ll be able to see elements’ styles, change them temporarily, see what effect they have on your layout etc.
Look into what a global reset can do for you.
You can add it yourself but codepen also provides a way to do it for you. Click on the cog icon in the upper left of the CSS editor and look at CSS Base.