As images are commonly transmitted or stored in compressed form such as JPEG lossy compression, image authentication demands techniques that can distinguish incidental modifications (e.g., compression) from malicious ones. In this paper, we propose an effective technique for image authentication which can prevent malicious manipulations but allow JPEG compression. An image is divided into blocks in the spatial domain, each block is divided into two parts by randomly selecting pixels, and average gray values for the parts are calculated. The average value is compared with that of the adjoining block to obtain an authentication signature. The extracted authentication information becomes the fragile watermark to be inserted into the image’s frequency domain DCT block. The experimental results show that this is an effective technique of image authentication.