You need to make ids unique. Also having 2 closing </a>
tags for the first block probably isn't good for you.
Try:
<a id="single_3" href="Foto's/symposium1_groot.png" title="Test">
<div id="symposium1"> <img src="Foto's/symposium1.jpg" alt="" /></div>
</a>
<a id="single_4" href="Foto's/symposium2_groot.png" title="Test">
<div id="symposium2"> <img src="Foto's/symposium2.jpg" alt="" /></div>
</a>