Just set the "default value" in the server behavior to be the current value in the database and then the value won't change when it is left blank since it will use the default. 
My page is doing the same thing. I set the default value, but it's still deleting the image name from my database if I don't upload an image.
I have attached a file with the pertinent information to see if anything sticks out at you guys. Dreamweaver CS5 and Universal Email 4.0.1
Thanks in advance,
Twitch


