[Therion] Transparent map-images

Hi All Michael and I are putting some large scanned map-images on our therion pdf plots, but near the point where they overlap, one always overwrites the other. We'd rather they be transparent so one map-image can be seen through the other. Will it help if I can figure out how to make the white background of the original scans transparent? Does anyone have any tips for making the white area of a jpg or pdf image transparent? I have access to Adobe Elements, but I'm not very good with it, and although there are instructions for doing this I didn't have much luck comprehending what was required. Regards Bruce