Few watermarking systems are designed to resolve the problem of joint ownership verification. This paper proposes a new watermarking system for proving joint ownership of digital images, where a TTP is needed. The watermarking key is shared by all co-owners using Shamir's secret sharing scheme as presented by Shamir (1979). Furthermore, each owner is distributed a detection watermark for individual verification. The simulation results demonstrate that the proposed system is effective, and it can resist malicious attacks performed by the co-owners and some common image processing operations.