This is fairly easy actually

You'll need to use a pc.
Download BlueStacks (Android Emulator). Then, download the snapchat app on bluestacks and sign in. Then, get a fake webcam application. I think ManyCam is a good one ? but I haven't tried it myself. These programs basically allow you to spoof your webcam, if youv'e ever been on omegle or chatroulette you'll have seen these being used to catfish. Most of them allow you to use a still image so you can just use the image that you want to appear on Bluestacks' front facing camera.
Hope this helped