Transmission of secret messages or images over the Internet using Shamir's secret sharing scheme has become popular. Some researchers use steganography with Shamir's method to hide noise like share images into natural looking cover images to improve secrecy. Stego images are authenticated against accidental or deliberate changes before recovering the secret. Authentication by a parity bit stream calculated by a keyed hash of stego images is commonly used. Researchers aim to increase the number of authentication bits to improve the authentication strength of their methods. Eslami and Ahmadabadi (2011) proposed a method with dynamic embedding strategy in 2011. They use a concatenated string of four bits, two from the current and two from previous block, to authenticate individual blocks. Even though chaining performs block based authentication, it cannot detect individual fake stego blocks and cannot authenticate the rest of the stego image blocks if it faces a changed block. This paper proposes a new secret image sharing method by selecting the number of authentication bits proportional to block size, contrary to Eslami and Ahmadabadi (2011) method which uses four bits to authenticate blocks regardless of block size. The proposed method has improved authentication for increased block size and can authenticate individual stego blocks as well. It produces good quality stego images and can still authenticate the rest of the stego image even after an altered stego block is encountered as shown in the experimental results. (C) 2012 Elsevier B.V. All rights reserved.