I guess I found the answer now. intesec recommended to use CCleaner, and those times that I have cleaned my harddrive first with CCleaner before saving a Ghost image the final size of the Ghost image has been about 6 Gb smaller, even though CCleaner only got rid of less than 1 Gb.
I have just saved one more Ghost image and today I forgot to use CCleaner first and the size of my Ghost image is 20 Gb.
I trust Ghost as it is now. I always use verify and the verify has always reported everything okay.
Jorgen Lyng