Background of a row can either be a color or an image. To add or change the background image of a row, in the squeezefunnels page builder point your cursor to the row you wanted to change it's background then click tool icon for Row Settings.
In the pop-up form select the type (color, photo, video etc.) of background you would like to add in the row.
Below click Select Photo and either select an image from your media library or upload a file > Select photo > Save