I created my own Photoshop template file to make the design and preview of the front & back images easier before importing them into H3Tool. All you do is stick your 834x1330 (size recommended by H3Tool) or larger image on the bottom layer and make either the "front" or "back" layer set visible to mask out the shape. Then it's easy to move around and adjust the image as needed, to fit around the screen & buttons, and within the edge flaps.
I stole a few of the images from the other template posted by saab_rider for the buttons and such.. hope that's alright. Anyhow, I made this for me, but hopefully other people will find it useful too
Download it here