If you're going to make a CP out of photos (this is what I did since I'm a horrible artist), I like photoshop a lot (I hacked together some hi-res photography from NASA, google images, etc..). I use the "Photoshop CS" version. However if you're going to be drawing something from scratch, it might be smarter to use a vector type program like Illustrator, since the results will be infinitely scalable, and you'll be working with a file around 100kb instead of 450mb like a large 300dpi CP image would be.