Watermarking an image using java

hey, im trying create a program with java that will watermark an image with a second picture, but i dont want the watermark to be visible, but at the same time, i want to be able to take an image with a watermark and extract the watermark to make sure that it is right.  Any pointers that could get me started would be greatly appreciated.

The problem with the popular implementations of steganography (e.g.: lower bits) is that the information is destroyed in a lossy medium (e.g.: JPEG).

However, why you'd want to reinvent the wheel is beyond me. Google.