上传图片尺寸超过某值时无法生成缩略图

测试商店插件时，发现上传的图片宽和高任何一尺寸大于500时，未能生成缩略图，如果宽和高度均小于500时，则可以生成，尝试修改($thumb[0]['file'], 500, 500)这一参数修改为更大值，均无效，求解，谢谢！

代码如下：

<div class="single-product-image">
                        <div class="single-pro-main-image">
                            <a href=""><img id="optima_zoom" src="{dr_thumb($thumb[0]['file'], 500, 500)}" data-zoom-image="{dr_get_file($thumb[0]['file'])}" alt="图片"/></a>
                        </div>
                        <div class="single-pro-thumb">
                            <ul class="thubm-caro" id="optima_gallery">
                                {loop $thumb $t}
                                <li>
                                    <a href="#" data-image="{dr_thumb($t['file'], 500, 500)}" data-zoom-image="{dr_get_file($t['file'])}">
                                        <img style="width:60px; height:60px;" src="{dr_thumb($t['file'], 100, 100)}" />
                                    </a>
                                </li>
                                {/loop}
                            </ul>
                        </div>
                    </div>