One other consideration in this mess is that the image you use for the transmap shouldn't be a JPEG; something about the compression scheme causes a variation from pure white near other colors. It's not visible to me when looking at the JPEG, but when I tried to map my graphics on the plane here using a JPEG, I got the ugly white border you're talking about. So, I saved the art file as a BMP and suddenly the lines were mostly gone. This is a non-reduced render straight out of RDS.