In your DFP Upload server behavior where you set the thumbnail image properties you set the name for it here as well. The default is to add in _thumb to the [filename] value. If you are renaming the thumb like this then you should be storing this value in your db in another column. How are you setting the name of the thumbnail in the upload server behavior?
If your thumbnail has the exact same name but is just in a different directory you should be able to use the same recordset binding as you are for the link but just specify the thumbs directory, just like in neilo's last suggestion.