1. A computer-readable memory device storing program instructions that are computer-executable to perform operations comprising: analyzing image content within each image of a set of images to determine at least one respective characteristic metric for each image, said analyzing including detecting whether an image includes edges or lines that are longer than a threshold length; determining a respective transition effect for each pair of a plurality of pairs of the images based on the at least one respective characteristic metric for each image the respective transition effect useable to implement a transition between the pair of images, said determining the respective transition effect including determining a region in each image of the pair to which the respective transition effect will be applied and including, when the edges or lines longer than the threshold length are detected in the pair, determining whether the respective transition effect to be applied involves the edges or lines longer than the threshold length; and storing an indication of the determined transition effects.