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.

>>13
>>3 HERE
PROTIP:
READ THE FILE FORMATS.
IT'S POSSIBLE TO PUT COMMENTS AT YOUR GODAMN IMAGES.
also it is possible to append text to the image without destroying it.

Are you on *nix ? try this

echo leethacker >> image.jpg
strings image.jpg | tail