Hi @Chrille,
You should use "EditRecord" prefix on Details page in order to reference a field.
So, you code should look like this:
<script type="text/javascript">
document.addEventListener('BeforeFormSubmit', function (event) {
var v3 = '<img src="http://image_url" alt="YES"';
var v2 = document.getElementById("EditRecordKontaktad").value;
if (v2 !="")
{
document.getElementById("EditRecordRingt").value = v3;
}});
</script>
Hope this helps.
Regards,
vitalikssssss