You don't need stepic for that.
>>> out = file("out.jpg", "wb")
>>> out.write(file("someimage.jpg", "rb").read())
>>> out.write(file("somehiddenfile.pdf", "rb").read())
>>> out.close()
stepic is something completely different it is for putting "really" hidden data into an image, whereas your copy
approach (and also my snippet above) just appends the file after the image's data. It is quite easy to extract the "somehiddenfile.pdf" from the generated file, whereas extracting steganographic information out of a real image is a lot more difficult.