For quick retouches, Photoshop is more than overkill--it's like going rabbit hunting with a nuke. All that bloat can take much longer than is necessary. Paint.NET, on the other hand, is an open-source editor with all the essentials, including tools to crop, rotate, resize images, adjust colors, and create collages. Unlike many free image editors, Paint.NET supports layers and has an actions manager. The pleasing interface boasts semitransparent windows for ease of use.
Paint.NET supports common image formats--JPEG, PNG, GIF, TIFF, and others--but not high-resolution RAW files. There are enough basic and intermediate effects and features to keep image-tweakers happy, though the red-eye removal tool is notably weak; those images may require manual attention. Way more advanced than Microsoft Paint, this is a must-have for even experienced digital image demigods.