Sometimes you just have to resize an image. Sometimes you want to edit a couple of images. Sometimes you have to do a complete overhaul of a ton of images.
Now, let me tell you the routine you have to follow; have Windows™, go to your favorite graphic editor, Adobe, website and buy or "buy" a copy of your favourite image editor, Photoshop.
But, let's say that the PC you have at hand is not capable of Windows, it could have Linux or, eventually, Mac OS X. Or say that you don't own the necessary amount of gold coins to buy (and not "buy") the upsaid software.
A simple, easy and straightforward solution is to search for another program. An example is GIMP, and to be honest it is the example of a libre/open source solution. It can work on Windows, Linux and Mac OS X and can handle the majority of use cases of the upcited Photoshop.
For this purpose, I proposed a workshop where I show the basics of the GIMP, divided in fived practical use cases.
An online version of the workshop presentation, is viewable here.
If you are interested, you can also download a zip containing presentation and examples.