You should be able to drag your pictures just like that to both iDVD and iPhoto.
In both iPhoto and iDVD there are menu options for adding pictures (instead of dragging).
Come to think of it: Did you actually create a file (JPEG or PICT) or did you merely create a still clip for use only in iMovie? (You would need to create an actual file to use in iDVD and/or iPhoto.)