It depends what programm you use and what you want to do But if you want to change for an example the colours of a picture the secret it's in opacity! (I don't make distinction between pros and noobs but well a noob image is smth that is ugly and a pro one is where you looks and everything looks nice, so yeah there are people who are called pro because they smudge and I call them noobs because they're like thousands of works that I've seen!) Where you have the difference between a so called "noob" and others is that noobs use a lot of green or blue all over the image and it's not balanced! And opacity allow us to harmonize!

And opacity exists in Photoshop and GIMP!

Btw in Deviantart there are great tutorial for photo changes (not banners just photos) whee you change the colours and look natural! I think that for GIMP it doesn't exists but I can make a small one later if anyone is interested.