1. A computer readable medium storing a media-editing application for execution by at least one processing unit, the media-editing application comprising sets of instructions for: receiving the addition of a video clip to a composite video project, the video clip having a sequence of video images at a first frame rate and the composite video project having a second frame rate for outputting video images; when the first frame rate does not match the second frame rate but is within a threshold of the second frame rate, generating output video images for a particular duration of the composite video project at the second frame rate by using each of the video images of the video clip once during the particular duration; and when the first frame rate does not match the second frame rate and is not within a threshold of the second frame rate, generating output video images for the particular duration of the composite video project at the second frame rate by using at least one of the video images for each output video image over the particular duration.