1. A computer-implemented method for selecting salient viewports, the method comprising: receiving, by one or more computing devices, a request for a particular image; identifying, by the one or more computing devices, a set of two or more potential viewports within the particular image, wherein each potential viewport of the set of two or more potential viewports is a sub-portion of the particular image; identifying, by the one or more computing devices, features within each potential viewport of the set of two or more potential viewports, the features being visual features; generating, by the one or more computing devices, a weight vector having a set of feature weights, each feature weight representing a weight of at least one feature; generating, by the one or more computing devices, for each given potential viewport of the set of two or more potential viewports, a feature vector having a set of values, each value of the set of values representing at least one feature, the feature vector for the given potential viewport indicating identified features within the given potential viewport; calculating, by the one or more computing devices, a score for each given potential viewport of the set of two or more potential viewports based at least in part on the weight vector and the feature vector for that given potential viewport of the set of two or more potential viewports; selecting, by the one or more computing devices, a viewport of the set of two or more potential viewports as a most salient viewport based on the calculated scores; and providing, by the one or more computing devices, the selected viewport such that only a sub-portion of the particular image corresponding to the most salient viewport is displayed to a user in response to the request.