Paint.net is a free software application for Windows.
It enables you to modify images and includes basic graphic design tools you can use in WordPress.
It includes some features similar to whats included in Adobe Photoshop however is far easier to use.
I use Paint.net mainly with featured images and screenshots to make new backgrounds which are a specific size and layer icons and other images over the main image.
Mac users looking for an equivalent to Paint.net should try GIMP.
Learn more about Paint.net