I suspect that the basic setting of the background color in the Helix ultimate template is not transparent, but white. If I do not enter a color in the section, the default color is #rrggbb and it should be transparent. But I think that in this case, if no color is specified, it is not transparent, but white. Presumably this is what it says in the template files. If I try to enter a transparency, e.g. (255, 255, 255, 0.1), the template converts the input to #FFFFFF. Therefore I don't think it is possible to display a background image behind the sections header, logo etc. because it is not transparent.