1. A method for managing images, comprising: selecting, from a storage device, an image included in an image library for which an image index is to be generated; determining at least one target region included in the image, wherein at least a part of a foreground of the image is included in the at least one target region, and a background of the image is not included in the at least one target region; extracting a set of one or more visual features from the determined at least one target region, the set of one or more visual features extracted from the determined at least one target region including a scale-invariant feature transform (SIFT) feature, wherein the extracting of the set of one or more visual features includes: for the determined at least one target region in the image: performing a SIFT to determine each of key pixel points in the at least one target region; and separately determining an N-dimensional feature vector corresponding to the each of the key pixel points, N being an integer; and for the N-dimensional feature vector corresponding to the each of the key pixel points: comparing a distance between the N-dimensional feature vector corresponding to a key pixel point and the N-dimensional feature vectors corresponding to the each of the key pixel points in a predetermined sample image library; determining a first key pixel point in the image library, the first key pixel point having a smallest distance from the N-dimensional feature vector corresponding to the each of the key pixel points; determining a number assigned to the first key pixel point; and forming a vector comprising the numbers corresponding to the first key pixel points determined for the each of the key pixel points, wherein the vector serves as the SIFT features extracted from the target region; determining, using one or more computer processors, a similarity value of the selected image and another image included in the image library based on the extracted set of one or more visual features; determining image categories to which the images included in the image library belong, based at least in part on the determined similarity value of the selected image and the other image; and assigning category identifiers to the images included in the image library, wherein a same category identifier is assigned to images belonging to a same image category, and different category identifiers are assigned to images belonging to different image categories.