As the existing block-wise lossless image authentication algorithms cannot determine appropriate sizes of image authentication blocks,a novel lossless image authentication algorithm with size-variable authentication blocks is proposed. In this algorithm,an image authentication block is composed of some basic image blocks,and the authentication code is generated by using a digital signature algorithm and is embedded into the authentication block via the lossless watermarking. Moreover,the robust transmission of the image ID is achieved via multiple embedding,which helps to distinguish different images and to resist the vector quantization attack. Theoretical analyses and experimental results demonstrate that the proposed algorithm can not only effectively resist the vector quantization attack but also improve the tamper localization accuracy because it can dynamically adjust the sizes of authentication blocks according to the characteristics of the host image and the fidelity threshold of the watermarked image.