Adobe PhotoShop. I have an old version of PhotoShop that I use to match the textures up to UV images from the models I assemble in the program listed below. Program cost-- A friend of mine gave me this copy, and I have guarded it like a watchdog ever since.) I wish I could afford the current version of PhotoShop which actually allows you to paint directly onto a 3D object. I've been thinking about subscribing to the Adobe Creative Cloud which would give me access to the current versions of PhotoShop and AfterEffects.
Adobe's Photoshop CS2 is available as freeware now.
Enjoy gratis with Adobe's blessing!
I'm not modeling, but thinking about book cover art. I was considering 3D modeling as a method for easier digital painting, like this stuff made with Daz 3D and Photoshop:
But after seeing these digital painting tutorials
, I might stick with digital painting for now! He can accomplish a great deal of realism with just a few very simple techniques you can understand quickly. If you want to use Photoshop like a paint canvas - and create almost anything you can imagine - this is a great hands-on starting point.