If you'd like the characters to have a painterly manga look, SAI has better support at your fingertips for brushes and color: http://browse.devian.../?q=shading sai , but I find an editor like Photoshop, or my favorite, GIMP, to be easier for more of an anime cel-shaded look since there's more robust layer support: http://browse.devian...r=9&q=cel shade Any of the tutorials on those pages can help you a lot better than I can in this topic.
Either way, try not to go too wild with blur and anti-aliasing, especially around the edges, when working on game art. It's a good idea to make a layer with just flat colors that you can select and do your actual highlights and shadows on separate layers; when you select a flat area of color to work in like that, you can be sure you won't 'scribble outside the lines' and you can mess with layer blends and oppacity later on. On the other hand, a painterly style relies a lot on mixing colors and brush strokes as you work, so those sorts of digital tricks can leave your painterly art looking too bland.
ED: By the way, a program like XnView is really helpful if you're just interested in converting image file types, changing color depth, resizing or other stuff like that. I run -all- my pictures through XnView when I've finished them to at least cut down on file sizes; it has a great exporter with lots of options for all the save formats and great preview function to see how it might effect the final product.
Edited by monkeynohito, 04 March 2013 - 03:59 PM.