GIMP, or the GNU Image Manipulation Program, is the FOSS alternative to Photoshop. Until the recent release of version 2.8, it could be quite a culture shock because its various toolbars were detached from the image window rather than part of a single window. Now you can select Windows>Single-Window Mode to get something that looks more like Photoshop. However, professionals who use more than one monitor usually prefer the classic style because they can have the image on one monitor and the toolbars on the other. GIMP comes with all the tools you need to create and edit images including photographs. While doing so, it stores the image in its own .xcf format and maintains a history of everything you have done so that, if you decide you don’t like an effect you have created, you can step back through the history and resume work on an earlier version.
GIMP can load almost any image format you offer it and will export to all the common formats; once you export an image, you lose its history. So, if you are likely to want to edit an image over several sessions, save it in GIMP’s own format until you are satisfied that you have finished.
If you are editing photos, you should do so in the order geometry, crop/resize, brightness/colour, sharpen, EXIF tags. Tools>Transform gives you the tools you need to adjust the geometry of the image and crop or resize it but you can also use the automatic crop options under the Image tab. The Image tab also offers a choice of colour mode while the Colours tab offers a wide range of ways of adjusting the colour and brightness of the image. But these simply scratch the surface of the options available with the Layer, Tools and Filters tabs offering many options for editing images.
For example, the Layer tab offers the options of adding layers to or editing different layers of an image or of masking all or part of an image both to allow adjustments to different parts of an image if, for example, one part is brighter than another and to permit a variety of special effects.
Finally, you can edit the EXIF properties with File>Properties>Advanced.
GIMP is available from http://www.gimp.org/downloads/. Scroll down for the Windows version.