I would like to make checkboxes containing images as where a client can give in which pdf's he would like to download and then submitting. Something like this: www.e-village.nl/downloads/onze-downloads/.

So the image part is not the problem but configuring the actual checkboxes themselves as this is databse driven i guess?

I guess the answer holds into the script on it.
Using fire bug I could see that the images with the "check button" holds like this:

<label class="downLoads download downloadBMQ342011">
<input type="checkbox" onclick="aTogle(this);" name="EVbenchmark2011Q34">
<span class="size12 darkgray">Benchmark Q3&4 2011</span><br>Trends en cijfers van het tweede<br>halfjaar van 2011
</label>

The rest is up to their script:

<script type="text/javascript">
function aTogle(elem){
var cleanClass = elem.parentNode.className.replace(' downloadActive', '');
if(cleanClass == elem.parentNode.className){
elem.parentNode.className += ' downloadActive';
}
else{
elem.parentNode.className = cleanClass;
}
}
function filledIn(form){
for(var i = 0; i < form.elements.length;i++){
if(form.elements.type && form.elements.type == 'checkbox' && form.elements.checked == true) return true;
}
alert('Geef aan welke downloads u wilt ontvangen, door ze aan te klikken.');
return false;
}
function countCheck(form) {var total = 0;for(var i = 0; i < form.elements.length;i++) {if (form.elements.type == 'checkbox' && eval("form.elements[" + i + "].checked") == true) { total += 1; }}form.aantal_downloads.value = (total); }
</script>