Make the image go behind the text and keep it in center using CSS

You can position both the image and the text with position:absolute or position:relative. Then the z-index property will work. E.g.

#sometext {
    position:absolute;
    z-index:1;

}
image.center {
    position:absolute;
    z-index:0;
}

Use whatever method you like to center it.

Another option/hack is to make the image the background, either on the whole page or just within the text box.

Make it a background image that is centered.

.wrapper {background:transparent url(yourimage.jpg) no-repeat center center;}

<div class="wrapper">
 ...input boxes and labels and submit button here
</div>

Well, put your image in the background of your website/container and put whatever you want on top of that.

Your container defined in HTML:

<div id="container">
   <input name="box" type="textbox" />
   <input name="box" type="textbox" />
   <input name="submit" type="submit" />
</div>

Your CSS would look like this:

#container {
    background-image:url(yourimage.jpg);
    background-position:center;
    width:700px;
    height:400px;
}

For this to work though, you must have height and width specified to certain values (i.e. no percentages). I could help you more specifically if you wanted, but I'd need more info.