Pixlr is the online version of photoshop :)

The one I use is Gimp. You can download it here -> (http://gofree.com/download/Windows-Software/Graphic-Design/gimp.php) or you can use photoshop. (But you have to download them) Its hard at first but you will get the hang of it the more you practice. And if you need help you can look up FanFic poster tutorials on how to make it. Well I hope that helped ^-^