first off, change the IDs for the div around the link and image upload, you are using the same ID as the elements themselves. the following code is based on using the IDs "SponsorsLinkDIV" and "ImageUploadDIV"
next, you need to add a variable for the ImageUploadDIV and another if statement to show or hide it based not he select value. I'm not 100% I have your exact requirements right, but this should get you on the right foot:
<script type="text/javascript">
function setTextField() {
var theList = document.getElementById("SponsorsType");
var theLinkDiv = document.getElementById("SponsorsLinkDIV");
var theUploadDiv = document.getElementById("ImageUploadDIV");
var theSelectedValue = theList.options[theList.selectedIndex].value;
//Show the link?
if((theSelectedValue == "Sponsor") || (theSelectedValue == "Premium") || (theSelectedValue == "Media")) {
theLinkDiv.style.display = "block";
} else {
theLinkDiv.style.display = "none";
}
//Show the upload?
if((theSelectedValue == "Sponsor") || (theSelectedValue == "Media")) {
theUploadDiv.style.display = "block";
} else {
theUploadDiv.style.display = "none";
}
}
</script>