I figured it out. The length dimension of a roll is not standard, and must be defined as a Custom size. I mistakenly defined my paper width as 1200 (thousandths of an inch) and my paper length as 2400, confusing image size with paper size. This didn't compute, so the printer defaulted to starting at the left margin. The paper width should be defined as 1295 (the maximum allowable width on the R2400) and paper length 2495 (just to keep things symmetrical). The image prints centered, just as it should. Life is good.