I will assume you made the same mistake I did. Did you create two seperate folders for the images and two seperate database fields?
That was my mistake. Youonly need the one database field for both the thumbnail and the original and any other versions of the image.
All you need to do is delete the '_thumb' or what ever the the default rename is set to so that each version you upload has the same name. Then when you call it up to display from the database you use the same field for all versions but you set your image source to go to the appropriate folder for what you are displaying.
So in short you end up with a file in each folder with the exact same file name which is what gets sent to the database.
Now that I am used to that, I kind of like that. Really does make sense. Hope that helps.