Images not centering

I just inserted an image onto a page and set it to be centered, but when I view the page, the image isn’t centered. I’ve set the image class as aligncenter and the css has the following for the aligncenter class:

.aligncenter {
		margin-left: auto !important;
		margin-right: auto !important;
	}

So why isn’t the image centered? Am I such a complete n00b that I’ve done something horribly wrong? (Probably…)

The page in question is my about us page.