It depends on which stage of prototyping you are ..

If you are still in sketching and ideation, Balsamiq is simple and to the point.

Once you have the idea designed, I recommend you hire a graphic designer, do the high fidelity images and buttons for you on photoshop , as , any way, you will need them for your developers too.

Whether you are at the balsamiq stage or the high fidelity images, I recommend you to go to invisionapp.com . I use it to show / demo simulate a full workflow from the images I have.

It will also help you discover gaps in your workflows that you didn't design, or what to improve in the workflow before you start working on implementation.