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.

>>6
its not so much that i need to reinvent the wheel as much as i need to implement said algorithm in a language that i understand.  >>3
no evil plans here, just a project for a class

If this is too complex, can anyone give alternative ideas for an an algorithm that is easy, but not too easy to implement, but has   practical use?