There are white spaces to the left and right of the image I added – how do I remove those?

Page Builder blocks are made up of Layouts, Columns and widgets. By default, every Column has 10.5 pixels of padding on the left and right. So if you’re adding a site-wide banner, that padding will make white space on the sides of your banner. To remove it, click your mouse in that white space to activate the Column in the Page Builder controls. Change the default left/right padding to 0 and then publish your changes.