in the anchor link add a url variable to pass the image ID, for example:
<a href="<?php echo $row_rsPromotionen['LinkToURL']; ?>?id="<?php echo $row_rsPromotionen['PROMID']; ?>">
in this example the image is passed as a GET variable named id
now open the binding panel, click the plus button and Select URL Variable, name the new variable id. this creates a binding for the id variable in the link.
in the update record behavior, set thew ID field to the PROMID column in your table, and bind the ID value to the id url binding you created.