Dealing with child window..

Hi ..

By default ... drupal displays the already Existing images in a 'dropdown' list.. (Attached images fieldset in editform of contenttype)..
Instead of tha I have created a 'text field' with a 'button' ..
Onclick of that button a child window called "imgpath.php" wil open listing the available image descriptions, created date, imagename and an 'ADD' button..
ONclick of a 'add' button that particular image's "fid" valud should get displayed in the textfield i have in the parentwindow..
I am able to retrieve that value.. but unable to set it to the textfield..
Wat i have to do.. I am totally struck..
Anyone kindly help me..