If you haven’t seen it yet: here‘s a little challenge I put together on my twitter page to see if people can find the
secret message in a pair of pictures. If you want to try cracking it then don’t read further because I’m about to tell the secret to it.
The trick is to set the last bit of every pixel to one of the bits for the hidden message. Most people wouldn’t
notice because flipping the last bit of a pixel only changes its color value by one, and most people won’t notice such a small color change, especially for a picture with a lot of color noise.
The advantage to this method is that it works in Gimp (and possibly Photoshop, I haven’t tested it), but you can only insert non-encrypted images.
Finally here is the link to the answer.