I usually use The Gimp for image manipulation. It's like Photoshop in that it's a complex interface designed for complex tasks. Most of my needs are simple though. They're more complex than what Microsoft Paint can handle but nowhere near what Gimp can handle. I recently tried Paint.NET after reading some good reviews, and I'm really happy with it. It's very easy to use and has a nice interface. All I've done so far is cropping, working with layers, painting in various colors, and adding text. Next time I need to manipulate an image, I'll likely to use Paint.NET instead of Gimp.