After you open a project in iDVD 6 that you created in an earlier version of iDVD, an alert indicates that photos used in slideshows, backdrops, or drop zones are missing. You may be prompted to locate the missing items.
To prevent this issue from occurring, update to iDVD 6.0.4 or later.
Out of all of the iLife apps I used back in the day, I think iDVD seemed the most magical. The fact that I could burn a DVD, complete with menus, on my Mac was so, so cool.