An electronic device having a display and a touch-sensitive surface displays, in a user interface, image(s) from a media item corresponding to a sequence of images in a display mode. In response to an input, the device displays an editing user interface that changes which images are included in the media item when the media item is displayed in the display mode, including concurrently displaying: an area display images in the sequence of images; another distinct area including representations of images in the sequence of images, a user-adjustable begin-trim affordance indicating a first boundary for playback through the sequence of images, and a user-adjustable end-trim affordance indicating a second boundary for playback through the sequence of images. The device further adjusts the media item, exits the editing user interface in response to an input, and displays the edited media item in the display mode in the user interface.