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.

>>3
the purpose of having a watermark that is not visible is that in case some fag site like ebaums takes an image and puts their watermark on the front, i could prove that i was the original creator by having my sig in the picture after i take the two apart.