Just out of curiosity, the backgrounds look like you may have modeled them. Is this the case? I know there are several 2D comic artists now working with SketchUp to create quick-but-accurate interiors and exteriors, that they then integrate as backgrounds with their hand-drawn characters ('hand-drawn' by way of a tablet, usually).
Nope, no SketchUp or any other kind of 3D. I did use the straight-line tool for the panel edges and parts of the backgrounds but it was all 2D.