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I don't know the internal format of a jpeg file and have no idea how the contents of a jpeg file can be changed without passing the file through special code that does know how to read and write the...

Look at the BufferedImage class. It has methods for getting and setting pixels.

I'm not sure how you can read and modify the contents of a jpeg file. I think the image has to be read into an Image object which allows access to the bytes defining the pixels.
Also I'm not...