When you scan the image with preview, The image
captured is a very low amount of ppi.
What you need is to not close the driver, But to do a full
scan after the preview scan.
A preview scan is only meant to check orientation and
position of the image, It's possible that your application
thinks you want to use the preview image when you
close the driver.
Try scanning an image at the highest possible
resolution and then scale down the image in an image
editing program that you import it to.
I always make a high resolution image and then save it
to a C.D., That's so that later on I can make copies at
any resolution I want without using the original.
Hope this solves your problem.