未捕获的TypeError：无法设置未定义幻灯片的属性'src'

Question

适用于Firefox但不适用于其他浏览器。

function slideshowUp() {
    num = num + 1
    if (num == 4) {
        num = 1
    }
    document.mypic.src = eval("img" + num + ".src")
    document.jenn.lopez.value = eval("text" + num)
}

我们从Chrome获得的错误：

  

未捕获的TypeError：无法设置属性＆＃39; src＆＃39;未定义的ps.html：39   slideshowUp ps.html：39（匿名函数）

<script language="JavaScript" type="text/javascript">
        var num=1
        img1 = new Image ()
        img1.src = "images/Lopez_Project_web.jpg"
        img2 = new Image ()
        img2.src = "images/Lopez_magazine_web.jpg"
        img3 = new Image ()
        img3.src = "images/collage_web.jpg"

        text1 = "Mask of Flight"
        text2 = "Training 101"
        text3 = "Collage"

        function slideshowUp()
        {
        num=num+1
        if (num==4)
        {num=1}
        document.mypic.src=eval("img"+num+".src")
        document.jenn.lopez.value=eval("text"+num)
        }

        function slideshowBack()
        {
        num=num-1
        if (num==0)
        {num=3}
        document.mypic.src=eval("img"+num+".src")
        document.jenn.lopez.value=eval("text"+num)
        }

    </script>
</header><br>
 <div class="c1">
    <a href="JavaScript:slideshowBack()"><img src="images/back.png" alt="Back" align="left"></a>
    <a href="JavaScript:slideshowUp()"><img src="images/next.png" alt="Next" align="right"></a>

    <img src="images/Lopez_Project_web.jpg" alt= "Mask of Flight"  id="mypic">
    <p>

    <form name="jenn">
    <input type="text" name="lopez" value="Mask of Flight">
    </form>

这是我在Firefox中运行的代码，但不适用于IE或Chrome