1. A method performed by data processing apparatus, the method comprising: receiving, by one or more data processing apparatus, a plurality of images and location data that specify a geographic location associated with each of the images; selecting, by the one or more data processing apparatus and based on the location data, a group of images that are associated with a geographic location that is within a threshold distance of a reference location, wherein the threshold distance of the reference location is proportional to a size of a geographic region that is represented by a particular map portion; identifying, as a reference image for the reference location and by the one or more data processing apparatus, one of the images that has at least a threshold level of similarity relative to a highest number of other images in the group of images; determining, by the one or more data processing apparatus, an image effect to use when presenting the reference image, the image effect determined based on the highest number of other images; and providing, by the one or more data processing apparatus and for presentation in the particular map portion, a photo collection image that includes a visual representation of the reference image and a visual representation of a number of photos under the reference image, wherein the number of photos is proportional to the highest number of other images.