I have experience doing this, i've worked 3 years an a half on a startup that have an app, and all the new features that they include have to have a prototype. I make all the buttons, text, dummy images, effects, animations, etc; and this protoypes then are showed to the programmers in order to make the final product. In some specific cases i create the prototype on Pixate to show especific controls that you can't show on a plain image. If you want, i can show you my Pixate prototypes or just the image version of it. Send me a message if you are interested!