bootstrap输入框前面有图标css

     <div class="vote_mode">
            <form id="vote_login" class="form-horizontal" onsubmit="return false">
                <div class="form-group top_30">
                    <div class="input-group btm_30">
                        <span class="input-group-addon"><img src="./images/user.png" class="icon"/></span>
                        <input type="text" class="form-control" id="username"  placeholder="请输入姓名">
                    </div>
                    <div class="input-group">
                        <span class="input-group-addon"><img src="./images/mobile.png" class="icon"/></span>
                        <input type="tel" class="form-control" id="phone"  placeholder="请输入手机号">
                    </div>
                </div>
                <div class="form-group top_30">
                    <div class="col-sm-offset-2 col-sm-10">
                        <button type="submit" class="btn btn-promise btn-block submit_btn" id="submit">登陆</button>
                    </div>
                </div>
            </form>
        </div>