1. A non-transitory machine readable medium storing an image-viewing application for execution by at least one processing unit, the image-viewing application comprising sets of instructions for: calculating a first average aspect ratio of a plurality of images for display in an image viewing area, wherein a first image in the plurality of images has a first aspect ratio and a second image in the plurality of images has a second, different aspect ratio; determining a number and size of regions of the image viewing area for displaying the plurality of images, the regions having the calculated first average aspect ratio, wherein each image is for display in a different one of the regions; and displaying the plurality of images in the regions of the image viewing area, each displayed image having a first same visual weighting, wherein the first image is displayed at the first aspect ratio and the second image is displayed at the second aspect ratio when a third image is added to the plurality of images, calculating a second average aspect ratio of the plurality of images, including the third image, for display in the image viewing area, wherein the third image has a third aspect ratio, different than the first and second aspect ratios; performing said determining and displaying for the plurality of images, including the third image, in order for each displayed image to have a second same visual weighting, wherein each of the first, second, and third images are displayed at the first, second, and third aspect ratios, respectively.