First of all I suppose you mean you got 2 columns in your MySQL DB one for big image and one for thumbnail.
You got 2 upload behaviors or u using multiple image thing to produce the 2nd image?
Are you sure both image filenames are in your database and files in server?
Lastly if all the above are correct, in order to show the images you insert an image placeholder where you want the image to show up, bind that column to it and lastly do not forget to add the path to the src of the image placeholder since your db has only the filename in it and not the path.
Please give more exact details of what you have done so I can help you further cause you are not giving enough info.
Which page you deleted the thumbnail? and why only thumbnail? you got 2 columns so you need 2 upload behaviors.