For those who don't have Photoshop or Gimp, try Pixlr. It's an online photo editor. It has all the tools you need to create a poster, just remember to save your graphic every time.  There is a tutorial thread here for pixlr editor. 

I was able to make this, my first pixlr poster, after I read this blog tutorial.

Note: Chrome froze on me for a second and then the eraser would no longer function, hence that white band near Kai's face. I could only save it as .jpg so just recolored the white band in Photoshop. But the rest was done in Pixlr editor.

If you don't have the patience to learn the basic tutorials here and experiment on Pixlr editor but you want to create your own poster, try Pixlr Express and make a collage. It's pretty easy to use and you'll get a decent looking poster.