By taking into consideration the detection performance， recovery quality and file size， this paper proposes an authentication-watermarking algorithm for an einvoice proof image in GIF format． According to the characteristics of an einvoice image invoice，all blocks are classified into an important block and an unimportant one． The authentication watermark of each block is generated according to the type code and content itself，and is inserted the least significant bit ( LSB) itself． To improve imperceptibility and filesize， the recovery watermark with unfixed length of each important block is generated and is embedded in the many unimportant blocks randomly chosen on the basis of secret key． The validity of an einvoice image is detected by threelayered detection by means of authentication watermark， type code and recovery watermark． The watermark capacity of the proposed scheme is reduced to about 0. 5 bpp ( bit per pixel) since the important blocks are used to generate the recovery watermark with unfixed length． The low watermark payload not only improves the quality of watermarked einvoice image，but also reduces the file size of watermarked GIF image． Furthermore， the tamper detection performance is improved due to the threelayered detection． Compared with the current similar watermarking methods，the watermarked einvoice image generated by the proposed scheme has a better quality，which means that its filesize is half in comparison with that generated by the current methods，without influence of the tamper detection and recovery performance．