High dynamic range (HDR) imaging is attracting an increasing deal of attention in the multimedia community, yet its forensic problems have been little studied so far. This paper proposes an HDR image forensic method, which aims at differentiating HDR images created from multiple low dynamic range (LDR) images from those created from a single LDR image by inverse tone mapping. For each kind of HDR image, a Gaussian mixture model is learned. Thereafter, an HDR image forensic feature is constructed based on calculating the Fisher scores. With comparison to a steganalytic feature and a texture/facial analysis feature, experimental results demonstrate the efficiency of the proposed method in HDR image forensic classification on whole images as well as small blocks, for three inverse tone mapping methods.