Are you passing the information from the select list to your page with the UE server behavior on it? If so you can add in a recordset to this page and filter it on the posted id for the image. Then you can reference the image from the recordset in the email message using the bindings.
If you need any more help with this please post back and include any further details about your setup and the message body, what you have tried, and what the current result is.