In iMovie, how do you show a clip of you talking and then a picture but your voice is still going on?

Lately on iMovie I've been trying to figure out how to do a certain editing trick. Here's an example: On the Bachelor, they do a lot of things where they will show clips of the bachelor staring off into the sunset but in the background Sean is talking about love. Then, they will continue Sean talking, but this time it's the actual clip of Sean saying all these things in an interview. Basically, I can't figure out how to edit the audio so that it is not showing someone saying the words the whole time, but at one point it will. I know this is kind've confusing, but if you know how to help, please do!