I think that digital photography is a two part history. Neal taught us a lot about the first part the other day. Shuttering, exposure, iso, light... and a lot of things that are to come are common (more or less) with traditional photography.
But this is a new era. We do not have films anymore, but bytes. After taking a photo there are a lot of things you can do: correct white balance, increase sky saturation or just transform them to a funnier photo.
Yes, Photoshop is the word. But as it lacks some useful characteristics (you have to pay for it, it does not work under Linux) so I will try to convince you about using the best alternative: GIMP.
There are tons of good information about Gimp, and most existing about Photoshop is somehow "compatible" with this free software. I suppose that the best entry point to this program is:
And if you want a free introductory book you can also look at:
I pickup Neal's chameleon photo and transformed a bit just to show an example of what you can do in just a few minutes. Next time I will teach you how to change someone's hair colour.