In this paper, we propose a novel watermarking algorithm that makes use of a generalized secret sharing scheme to address the problem of joint ownership for digital images, which has not be adequately addressed so far. Given that multiple owners create an image jointly, only an authorized group of owners are given distinct shares and can create watermarks jointly. Then, each authorized owner?s contribution to the watermarks is put in the different sub-bands in the wavelet domain of an image so that the watermark detector can verify joint ownership as well as partial ownership. Experimental results show that the proposed algorithm is robust against a wide range of digital signal processing procedures.