Introduction to Steganography

Steganography is the art and science of writing hidden messages in such a way that no one, apart from the sender and intended recipient, suspects the existence of the message, a form of security through obscurity. The word steganography is of Greek origin and means "concealed writing" from the Greek words steganos(στεγανός) meaning "covered or protected", and graphei(γραφή) meaning "writing".
Source:Wikipedia
Further reading:Steganography

Hide file in Photo

How to hide a file in Photo/Image

Want to hide any executable file in an image? Hiding a file in an image is very simple process. All you need to have a computer (windows/Linux) and some basic knowledge.
Requirements
1. The image (image.gif) file into which we'll be hiding our file.
2. Files which we'll be merging with the image file.
Procedure
Now before we start compress your files (which will be merged with image) to file.zip.
WINDOWS
Open command prompt and move to the folder (by cd command) where the files are present (image.gif and file.zip) and type the code:
copy /b image.gif+file.zip newfile.gif
Linux
Open terminal. Move to the directory, where both files are present. And type the following code:
cat image.gif file.zip

>

newfile.gif
Done! If you open the file with image viewer the file will act like an image. Now there are two ways to access the files either change the extension of new file to .zip or open the file with a zip extractor.