Image link question

[Stormraven]

Im creating my own style and we coded it but for some reason vbulletin is playing up and I cant understand why, here is the css code for the main image.

#logo {
background: transparent url(images/misc/banner.png)top left no-repeat;
position: relative;
float: left;
height: 110;
}

and here is the other bit, the html div:

<div id="logo">
 <a href="/"></a>
</div>

for some reason that wouldnt work,, to get it to work we had to put the image code there again for some reason, like this:

<div id="logo">
 <a href="/"><img src="images/prowebforums/misc/banner.png" border="0" /></a>
</div>

I don't understand why we have to declare the main logo twice, its nothing big but I just want to understand why it's doing this.