I have a custom List with a dropdown list field called "Rate" with two
values = Thumbs Up and Thumbs Down.

I have another icon field (picture field).

When the user selects Thumbs Up, I would like to set the image Like.jpg to
the icon field and when the user selects Thumbs Down, I would like to set
the image disLike.jpg to the Icon field.

This is what I'm doing on the NewForm.aspx using SharePoint Designer.

<script>
bunch of code.......

if (theType.value=="Thumbs Up")
{
alert("TU");
document.forms("ContentForm").elements("ows:icon:url").value = "http://
-------/ImageLibrary/Like.jpg";
document.forms("ContentForm").elements("ows:icon:desc").value = "Thumbs
Up";
}
else if (theType.value=="Thumbs Down")
{
alert("TD");
document.forms("ContentForm").elements("ows:icon:url").value = "http://
---------/ImageLibrary/disLike.jpg";
document.forms("ContentForm").elements("ows:icon:desc").value = "Thumbs
Down";
}

</script>