I have designed my own currency. It's basically a very fancy I.O.U. seeing as how it's backed in US currency.
I did most of the front work in 2005, then re-did a bit and designed the back in late 2007. The whole thing is designed at 600DPI using NO anti aliasing, and only three colors in separate layers. When printed on high settings one layer at a time on good paper, it looks very much like real currency.
The rather phallic structure on the back is Wardenclyffe Tower as it would appear if it had been completed. It was part of Nikola Tesla's system that was intended to broadcast electric power. You can't tell in the picture, but all the brickwork and details are actually visible on the printed note. It took a VERY long time to do this.
This one shows how it looks at actual size on the screen. The size of the image allows for great detail when printing. Even lines mimicking micro-print are possible on good printers. Please understand that I'm not promoting counterfeiting, just interesting techniques for creating artwork. Currency designs and computer graphics have always interested me.