We present a new watermarking scheme for joint ownership verification of digital images. In the proposed scheme, the watermarking key is held by a trust third party (TTP), and it is kept secret to all co-owners using Shamir's secret sharing scheme. Each owner is assigned a share of the watermarking key and a distinct detection watermark. The scheme can provide individual and joint watermark detections, and the robustness of individual watermark detections does not decrease with the increase of co-owners. Experimental results and analysis demonstrate that the scheme is effective for joint ownership verification and is robust to common image processing operations.