Bug trying to unmerge images with JPG format

[kelvins]

The unmerge function is not working with JPG image formats, for example:

$ python steganography.py --input_image1 res/image1.jpg \
                          --input_image2 res/image2.jpg \
                          --output_image res/merged_image.jpg

$ python steganography.py --input_image1 res/merged_image.jpg \
                          --output_image res/unmerged_image.jpg

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