"can i add the file of pictures to the same DVD"
No, you cannot burn more files to it if you have finalised (closed) the DVD.
"to copy a picture from it, can he access the file with all the pictures"
To explore the contents (see list of folders and files) on a DVD:
Hold down the 'Shift' key and put the DVD in the tray. Keep the 'Shift' key held down until DVD drive powers down. (holding down the shift key stops the DVD from auto running)
Now you can open Windows Explorer and look at the folders and files on the DVD and copy any you want to.