1. A non-transitory 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 an 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 differs from 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 mapping each video image of the video clip to a different single output video image in the particular duration; and when the first frame rate differs from the second frame rate and is not within the 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 of the video clip to generate each output video image in the particular duration.