语义UI Css图像对齐

Question

我有一个问题...我无法将我的徽标（logo_tool.png）图像居中，它位于左侧。为什么？请帮忙

<div class="pusher">
    <div id="top-menu" class="ui inverted <?php echo $general['layout_theme']; ?> labeled icon menu">
        <a class="item" ng-click="toggleSidebar()">
            <i class="sidebar icon"></i>
            <?php if(isset($main_menu['top_menu_button'])) echo $main_menu['top_menu_button']; ?>
        </a>
        <a href=""><img class="ui medium centered image" src="/images/logo_tool.png""></a>

        <?php if($donation['enabled'] && $donation['top_menu_show']) : ?>
            <div class="right menu">
                <a class="item" ng-click="openDonationModal()">
                    <i class="bitcoin icon"></i>
                    <?php echo $donation['top_menu_button']; ?>
                </a>
            </div>