Most designers use Photoshop for their image editing. We have a short Tutorial on Photoshop that will get you started.
There are some other alternatives.
Gimp is a freeware replacement for Photoshop. It's probably the best of Photoshop's competitors, but not as good as the real thing. Still it's free, and in some situations this is what we need. You can download and install Gimp on Mac Windows or Linux.
There are also some online services that allow you to edit your photos without having to install anything.
Picnik is a piece of software that runs together with Flickr, Yahoo's image storing service. Basically to use it you upload your images and then edit them online. http://www.flickr.com/help/picnik/
Pixlr is another online editor, that seems a lot better than Picnik and doesn't cost anything to use. Try www.pxlr.com. The interface looks pretty familiar for anyone who knows Photoshop. One problem: some of the menus don't display on the Mac. If you are on a Mac, try it and you will see what I mean. It's still usable but sometimes you have to guess which button is "OK" and which one is "Cancel". It sort of works.
Splashup is another online picture editor. I haven't tried it in detail but it looks interesting. http://www.splashup.com. You have to sign up for an account. It seems to work. Notice that the color picker forground/background is reversed from Photoshop; lower right is the pen color.
There is an online version of Photoshop, that I have not tried. http://www.photoshop.com. I didn't try it because it seems to have problems dealing with the Mac. It wants you to sign up for a free account, but their signup page didn't work on my computer so I could not sign up. Perhaps I will try it later from a windows computer.
There are a number of other image editing programs you can use. Here's one list:
http://sixrevisions.com/graphics-design/10-excellent-open-source-and-free-alternatives-to-photoshop/