Based on reading several posts, I opened the non-rendering original files (photos) in Photoshop, re-sized them (one was particularly huge) re-saved them with new names, re-imported them into FC and replaced the clips in the timeline. They rendered normally, but when I tried to output the file (to make a master for the DVD) FCP “quit unexpectedly.” So I did a system re-boot, opened FCP, created a new timeline, and copied and pasted the contents from the old to the new. It worked.