When making an image from a folder, DropDMG copies to the image the background pictures and view options of all the contained folders. It does not copy this information for the top-level folder because Apple does not provide a way to do this. If the top-level background picture and view settings on the image are important to you, you can set them as follows:
Some people like to save the background picture file in an invisible folder named .background, so that it is not visible on the mounted image. This folder will not be visible in the Save window of your graphics program. You can, however, type Command-Shift-G. This will let you enter the path of the .background folder so that you can save the picture in it.