It's because of the file type you're saving it, it's creating all those little artifacts, I believe that's what they're called.
You can erase them or color over them there's no other way to get them off :/
It happens when you save it as a jpg. If you save it as a bmp and then finish it in another program like GIMP, then it should be fine once you save it as a jpg, but you must save it in GIMP or another program.