The steghide application for Linux allows a user to embed a file inside an image that may then be retrieved by another person who knows the passphrase. The example below shows how to embed one image inside another. I am sending a secret image, a Gnome icon of an AC adapter and I am embedding it into an Xfce4 wallpaper. Considering the file size differences, this should hardly be noticeable.
[jason@localhost Documents]$ steghide embed -cf xfce-blue.jpg -ef ac-adapter.png Enter passphrase: Re-Enter passphrase: embedding "ac-adapter.png" in "xfce-blue.jpg"... done
And this is how to extract the image at the receiving end. Once the passphrase is entered, the image will be extracted.
[jason@localhost Documents]$ steghide extract -sf xfce-blue.jpg Enter passphrase: the file "ac-adapter.png" does already exist. overwrite ? (y/n) y wrote extracted data to "ac-adapter.png".
There is much more information about the steghide command here: http://repo.hackerzvoice.net/depot_madchat/crypto/stegano/windos/steghide.txt. But this guide should get you started.