subtitles over text in iMovie???

I don't see how I can place subtitles over video or still images. I have tried clip to characters and subtitles while having the video in the timeline selected. However, it doesn't allow me to hit update. I can see the still image with the multiiple subtitles running over it in the preview box, but I don't see how I can add it into the movie.

Any help is greatly appreciated! Thanks!