Page Builder allows you to divide your rows into multiple columns / cells. Many users have trouble when it comes to styling cells, simply because this set of options appears hidden and if you're new to Page Builder you easily miss this useful feature.
In order to access cell styles you need to edit the row and click on the cell you want to edit. A set of options (Cell Styles) will appear under Row Styles allowing you to customize the cell.
If you want to change the background image of the second cell (girl on the right), you can do it by clicking on the second cell inside row edit mode. This will instantly bring up Cell 2 Styles and you'll be able to change the background image under the Design panel.
It's important to note that Cell Styles cannot be copied from one cell to another, so keep in mind that if you change to order of the content (E.g. have the large image on the left and the testimonials on right), you'll need to manually adjust the cells to fit the new layout.