Digital watermarking techniques sign images by introducing changes that are imperceptible to the human eye but easily recoverable by a computer program. Generally, the watermark is a number which identifies the owner of the image. The locations in the image where the watermark is embedded are determined by a secret key. Doing so prevents possible pirates from easily removing the digital watermark. Furthermore, it should be possible to recover the embedded watermark from an altered image. Possible alterations of signed images include blurring, compression, and geometrical transformations such as rotation and translation. These alterations are referred to as attacks. A new method based on amplitude modulation is presented. Single watermark bits are multiply embedded by modifying pixel values in the blue channel. These modifications are proportional to the luminance and either additive or subtractive, depending on the value of the bit. This new method has shown to be resistant to both classical attacks, such as filtering, and geometrical attacks. Moreover, the watermark can be recovered without the original image.