IE8 shows the images properly - as does firefox... The problem with IE7 is because you have a border on #centerboxwrapper... remove the border to fix the problem...

alternatively, edit the product_listing.php module (in your customer module folder)

if (PRODUCT_LISTING_LAYOUT_STYLE == 'columns') {
if ($num_products_count < PRODUCT_LISTING_COLUMNS_PER_ROW || PRODUCT_LISTING_COLUMNS_PER_ROW == 0 ) {
$col_width = floor(100/$num_products_count) - 0.5;
} else {
$col_width = floor(100/PRODUCT_LISTING_COLUMNS_PER_ROW) - 0.5;
}
}

the subtraction of -0.5 from the calculation should help IE7...