Okay, so first off, we'll start with the link to a free, downloadable version of Photoshop CS5. And guess what? It's not going to harm your computer.
Next, you'll need PSDs.
Wait, let me explain to you what a PSD does in a picture.
Can you guess which side has the PSD on this picture? Hehe. I'm sure you can tell that the brighter side that makes colours "pop" sort of is the side WITH the PSD. They can enhance a picture, and it doesn't take much. On this one, (this is one of my PSDs) I just used three adjustments: Brightness/Contrast + Curves + Vibrance.
Basically, a PSD makes something prettier.
That's all I can think or right now, but I'll add more when I think of it.