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.

hey, im trying create a program with java

i've stop'd reading there.

You're a dumb fuck, you want a watermark that is not visible?
hint: that's not a watermark.
That's steganography.
You can either
1) append to the image
2) read about the images format and use it for your evil purposes
...
>_>