I embedded the CSS in my web page and works fine. But I try to make it as an external CSS, it won't work at all. Just don't know how to fix the problem!

```
<html>
    <head>
        <style>
            #cellPic{
            background-image: url("./pics/bird.png");      
            }
        </style>
    </head>
    <body>
        <table id="cellPic">
            <tr>
                <td id="cell0">0</td>
                <td id="cell1">1</td>
            </tr>
        </table>
    </body>
</html>
//External CSS named style.css:
#cellPic{
    background-image: url("./pics/bird.png");  
}
//My web page with the external CSS (won't work)
<html> 
    <head>
        <link rel="stylesheet" href="css/style.css"/>
    </head>
    <body>
        <table id="cellPic">
            <tr>
                <td id="cell0">0</td>
                <td id="cell1">1</td>
            </tr>
        </table>
    </body>
</html>

```

Thank you in advance for any helps...