We need to see the css and html otherwise we are just guessing.:)
Do you have a full doctype?
Have you cleared your floats (i.e. is the image snagging on a float previous to it).
Is the image display:block or display:inline? If its display:block then margin:auto should center it. If it's display:inline then text-align:center on the parent should center it.
Or it could be a number of other things