根据从下拉菜单中选择的选项更改页面上的图像时，我遇到了脚本问题。当我使用一个下拉菜单更改一个图像时，我可以使其正常工作。但我正在尝试扩展代码，以便可以从一个下拉菜单中替换两个图像，但我不确定我的代码到底出了什么问题。

<script>
window.onload=function()
{

    bp='http://www.nessasneedles.co.uk/images/layout/shop/samples/fabric/', //base url     of your images
    imgnum=4, //Number of your images. This should match on your comboboxes options.
    thumb1=document.getElementById('outer_example'), //id of your outer image that will     be changing. The outer of the bag
    thumb2=document.getElementById('lining_example'), //id of the image using in the if     clause for second image
    combobox1=document.getElementById('outer_option'), // id of your combobox. The     select box for the outer design.

    combobox1.onchange=function()
    {
    thumb1.src=bp+'img'+this.value+'.jpg';
    
    if (this.value = "Cats") {
        thumb2.src=bp+'img'+"Purple Gingham"+'.jpg';
    } else {
        thumb2.src=bp+'img'+"Pink"+'.jpg';
        };    
    };
        

}
</script>