1. A computer readable medium storing a program causing a computer to execute a process for adding image identification information, the process comprising: calculating first feature vectors for partial regions selected from a target image to be processed; and adding a piece of first identification information indicating content of the target image to the target image using a group of decision trees that are generated in advance on the basis of second feature vectors calculated for partial regions of a learning image and a piece of second identification information added to the entire learning image, wherein the adding includes deciding the piece of the first identification information for the target image in order to add the piece of the first identification information to the target image by multiplying a prior probability of a piece of the second identification information by a ratio between a first product and a second product, the first product being calculated by multiplying likelihood functions obtained from a proportion of the number of pieces of the second identification information that have reached individual leaves of the group of decision trees with respect to the total number of pieces of the second identification information, the second product being calculated by multiplying prior probabilities of the calculated first feature vectors, when a group of the second feature vectors and a group of the pieces of the second identification information are supplied to the group of decision trees to calculate the first and the second products.