If all you want is a quick image retouch, Photoshop is overkill. Paint.NET, on the other hand, is an open-source freeware editor with all the essentials, including tools to crop, rotate, resize images, adjust colors, and create collages.
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.
Unlike most free image editors, Paint.NET supports layers and has an actions manager. The pleasing interface boasts semitransparent windows for ease of use.
Version 3 fixes many old quibbles, including a convenient thumbnail manager for handling multiple image editing, and improved program graphics. Paint.NET is far more advanced than Microsoft Paint, and given its free price, it's a must-have for anyone needing to fix images.