You'll have to remove the red part here:
#productMainImage, #reviewsInfoDefaultProductImage, #productReviewsDefaultProductImage, #reviewWriteMainImage {
margin:10px; text-align:center; line-height:20px;
and fiddle with the width here, probably after breaking out productMainImage to it's own statement:
.centeredContent, TH, #cartEmptyText, #cartBoxGVButton, #navCatTabsWrapper, #navEZPageNextPrev, #bannerOne, #bannerTwo, #bannerThree, #bannerFour, #bannerFive, #bannerSix, #siteinfoLegal, #siteinfoCredits, #siteinfoStatus, #siteinfoIP, .center, .cartRemoveItemDisplay, .cartQuantityUpdate, .cartQuantity, #cartBoxGVBalance, .leftBoxHeading, .centerBoxHeading,.rightBoxHeading, .productListing-data, .accountQuantityDisplay, .ratingRow, LABEL#textAreaReviews, #productMainImage, #reviewsInfoDefaultProductImage, #productReviewsDefaultProductImage, #reviewWriteMainImage, .centerBoxContents, .specialsListBoxContents, .categoryListBoxContents, .additionalImages, .centerBoxContentsSpecials, .centerBoxContentsAlsoPurch, .centerBoxContentsFeatured, .centerBoxContentsNew, .gvBal, .attribImg {
width:850px;
margin-right: auto;
margin-left: auto;
}