I've been interested in digital painting for a while now. I tried doing it with a mouse but never got anywhere. I then got a little bamboo tablet but had some trouble figuring out how to do more or less photorealistic art. The process I use for illustration just did not work for painting or digital otherwise. I looked at other tutorials and got hints at what I was doing wrong or more accurately how I was thinking wrong. This is all about what I've learned.
This kind of art can be done with a range of software from GIMP to Photoshop. You'll need a program that uses layers and has a smudge tool. The difference between programs is significant but the method here can be worked on any of them.
At a minimum a tablet is important to keep the feel of drawing. I have a small Wacom Bamboo but I was able to get a hold of a old Fujitsu tablet that has a built in Wacom. Being able to draw on the surface that the artwork is on is a huge advantage but I've done this on my Bamboo also.
This is a raw outline of how I've been painting. I haven't settled on any one method yet, you probably won't right away either. Be willing to practice and experiment to find your style and a process that works well for you.