this could be answered in another thread, but in order for the DataAssist update not to erase the image from the record, make sure to set a default file in the Upload server behavior. In your example you had something like this:
'DefaultFileName' => "",
but it should be:
'DefaultFileName' => "".$row_WADAmap_Advertisers['logo_image'] ."",
Let me know if you still have any other questions.