You would need to rip the video off of the DVD as a video file, then add the pictures you want and reproduce it. So no you can't edit the DVD, but you can take the video off the DVD, edit it and reproduce it and reburn it to a DVD.
There may be a tutorial somewhere for this (I don't know) however, if you open Power Director, insert the DVD that you want to rip the video off in your DVD drive on your computer, once it is loaded, click on capture (at the top left of power director). Choose the option AVCHD and it should show the opening thumbnail of the video that you are wanting to capture. Under capture preferences you can choose different scenes (if there are different scenes) however, if you check off Movie, it will check all of the different chapters. At the top under the screen showing your video, you can then click the record button (button with the red dot in the middle) You will receive a pop up that says extracting. When it is done on the top right you will see a video (this is the capture from your dvd). When this is done, click on edit and it will load into the timeline for editing.
Hope this helps.