I've already talked about Illustrator. I'd like to talk a bit about Adobe Photoshop and Corel Painter. Photoshop and Painter differ from Illustrator because they are primarily raster based packages (not vector based like Illustrator) If you are interesting in making images that mimic traditional oil painting or other traditional painting mediums Photoshop or Painter may be a better program to start with than Illustrator. Also if you are interested in manipulating existing images and photographs (maybe collages?) I would also recommend Photoshop or Painter and not Illustrator.
The following is a piece of mine will give you an idea of the smudgy painterly style that you can accomplish using Photoshop.
Photoshop is my tool of choice. I purchased Painter years ago (version 7) and found it ran REALLY slow on my PC compared to Photoshop so that's why I gravitated toward Photoshop. But I've been told Painter it is much better now and in fact I just received my brand new Painter-X CD in the mail last night and I'm going to give it another chance.
Both of these packages let you use brushes and smudge colors in a way that more closely minics the use of real paints. But which one is right for you?
Photoshop is an industry standard tool that has very large user base and very active support forums. It's also a very stable and robust piece of software. I'm amazed an how stable it is. Every publisher I've worked with has been able to accept and sometimes prefer to receive images in Photoshop file format. And there are gobs and gobs of books on the market about how to use Photoshop.
On the down side, Photoshop's primary function is to manipulate photographs. I believe the number of users that use photoshop strictly as a painting tool is a rather small percentage. Many of the new features that are incorporated into the photoshop are more geared to photographers not painters.Painter on the other hand is designed for digital painting. There are a mind boggling number of options for fine tuning your brushes and papers. Many more than Photoshop has to offer. But there isn't as large of a user base and not nearly the number of after-market books available.
There are many ways to use these tools. One approach is to mimic traditional materials, such as watercolors, pastels or oils. I think watercolors (especially wet, drippy watercolor) is hard to replicate on the computer. But with other mediums, it can be tough to tell if an image was done digitally or with good old paint and turpentine. How much can these tools mimic traditional media? Check out these artist's websites and you be the judge...
Here's a photoshop piece by Brandon Dorman. Check out the rest of his website. I don't know if it's all Photoshop but it IS all wonderful.
Ryan Church's website is another totally amazing artist that uses Painter to create beautiful concept art. Hmmm, according to his website, he has a DVD out explaining his techniques (I think I gots to get me one of those!)But you don't just have to copy traditional ways of working. Going digital also opens up a new realm of possibilities. Some people feel that a digital image is only successful if you can't tell that it's digital. I don't subscribe to that theory. Yes, there is a lot of bad digital art out there (there was a lot of bad art done before computers too) But the choice of medium doesn't make an image inherently bad.
Check out Scott E. Franson's work for instance. His lovely children's illustrations (done using Illustrator and Photoshop) definitely have a digital feel to them but they are also sweet and fresh looking.
Hopefully I gave you some ideas about where to start exploring the world of digital art and maybe inspired you to start playing with some of these tools. Because, above all, digital painting is really fun (and it doesn't stain your clothes!)