Joachim is right about picture size: the displayed size should match the original width and height (this case, since it’s a 2 picture matrix, half size should be appropriate) most of the time. The original picture looks jagged if I just open it in Finder Preview on Mac. It’s more of a picture editing question than GDL, I think. We got the Hoods right in the Library, but if you take a closer look to the door handle popup images, you can see some jagged lines there as well (they are not prefect at all). You may try to save a double sized original picture with better resolution (so it has smaller visible pixelation at the slanted edges), and scale it to half size in the command’s image_x, image_y parameters. That may help a bit. Using .png format is recommended as well instead of .tif.
Library Developer, Library Team