In this paper, a novel semi-fragile watermarking scheme for image authentication is proposed, which addresses the issue of protecting images from illegal manipulations and modifications. The scheme extracts a signature (watermark) from the original image and inserts this signature back into the image, avoiding additional signature files. The error correction coding (ECC) is used to encode the signatures that are extracted from the image. To increase the security of this scheme, user?s private key is employed for encryption and decryption of the watermark during watermark extraction and insertion procedures. Experimental result shows that if there is no change in the obtained image, the watermark will be correctly extracted, thus will pass through the authentication system. This scheme is tolerant of lossy compression such as JPEG, but malicious changes of the image will result in the breach of the watermark detection. In addition, this scheme can detect the exact locations, which are illegal modified blocks.