This is the science of steganography. There was a lot of research done about 6-7 years back in this field, and a lot of free software available. Hell, you can even write your own software to encode data in images. It is quite simple - altering a few bits in an image file will corrupt the image a little, but this can be controlled depending on the bits and bit patterns you alter, and on how much you alter. For practical purposes, it will still look like a valid JPEG or GIF to a casual observer but since the receiver also knows the algorithm, he will be able to decode it and read the hidden message.
As for proporta, I would really hesitate to use it, after reading such statements on their website : "We don't have a 'bit strength' measurement to tell you and we're not sure how many 'ages of the universe' it would take to break. Quite simply, we don't know how secure it is"
