Another free program I downloaded recently is ArtRage. ArtRage is a free painting package that lets you play around with realistic painting tools in a simple, intuitive environment. ArtRage lets you paint your own picture from a blank canvas, or load an image to 'trace' using any of its painting tools, letting you recreate your own images as paintings. Not bad for free.

Another thing I would recommend is getting a graphics tablet if you don't already have one. They come in a variety of sizes. It's more natural to draw with a pen shaped tool than a mouse and with the pen you also have pressure-sensitive control of your brush in Photoshop or any other software.