Having read through the linked message, I don't find my method, so I'll pass it along. Make sure the catalog thingee is showing on the right side of the screen. Click on the background color selector of the main screen and move the arrow over to the one of the thumbnails that isn't selected (ie., that isn't highlighted). This will set the background to the same color as the thumbnail and it will disappear when you export image, import image as background,and then move the object out of the camera range before you add it to the catalogue screen. Turn off ground shadows is a good idea.