Is steganography valid only for digital images or also for printed images? If I embed some information in an image using steganography and then I print that image, will I be able to retrieve information from the printed image or not?
It depends on the steganography method you're using, and on how the image is printed.

Most steganography methods designed for digital images probably won't survive printing and scanning the image, or at least doing so will introduce a significant amount of noise into the concealed data that will exercise the error correction pretty hard and significantly limit the maximum payload capacity.

However, there are also steganography and watermarking methods that are designed to survive printing, or even designed specifically for printed images.  Indeed, you may have already unknowingly included such watermarks in your prints, if you've printed them on a printer that embeds a machine identification code in its output.