This paper aims to design robust correlation-based watermarking methods for images. We first present a model of watermark embedding and extracting processes and carry out their analyses. Then we examine the robustness of the watermarking method against image processing and clarify the reason why detection errors occur in the watermark extracting process. Based on the result, we improve the watermark extracting process and design robust watermarking systems. The improvement is accomplished using the deconvolution technique. Numerical experiments using the DCT-based watermarking system show good performance as expected by us.