Does your map have a grid burned into the image, if so they are likely 2 or 3 pixels wide which make the overall thing difficult to fit to the Roll20 single pixel grid lines. You are best off getting a good fit on one plane and then trying to get the other plane to work with minute adjustments, this might mean adjusting the original plane as well occasionally. Once you've got it close enough you could either turn off the Roll20 grid and use the map image one or make it transparent for snapping purposes and enjoy the 'close enough' experience lol.