10 pixel Shift Website

I centered some objects on a website, added some images, horizontal lines, etc. Everything works just fine but their remains some kind of 10 or 20 pixel margin on the left that I can´t edit. My centered Images are thatswhy 10-20 pixels too much on the right an an horizontal top border of a div that has 100% width does only reach from the right end of the screen to 10-20 pixels before the left end of the screen. Do you have any idea whats could be wrong with my website?