Actually I like Photoshop. It IS expensive, but you can purchase cheaper versions of it which are perfectly capable. It does take a little time to learn, but if you can master the keyboard shortcuts it helps a lot. It really can do everything - from HDR to 3d painting.
There are loads of Image editing applications, including the free one GIMP. Its personal preference. GIMP is very capable, but I'm not personally keen on the interface, even if it is a bit like Photoshop. It also runs on Linux.
In windows, its useful to have the application set up so that a right click on the image will launch the application, or open the image in it.