Another software to add to the list is Adobe Fireworks. A video worth reviewing is:
Something important that I picked up in the video, is not to reinvent the wheel. There are plenty of already pre-made visuals out there, so It would be a big mistake to be making your own.
Some templates that I like to use (all free):
These are great high quality, to scale, photoshop vectors = save time and have fun.
Another video on fireworks that covers how to create interactive prototypes and wireframing: