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 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 in the first image. I could only save it as .jpg so just recolored the white band in Photoshop [second image]. 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.

The toolbar is at the bottom.

1.Click Layout and choose the format you want, and play around with the values for spacing, roundness and proportion. 

2. To insert your images, move your cursor on top of a box and click on the plus sign.

3. The swap, edit and delete icons appear when the cursor is above the image. 

4. You can edit individual photos by clicking the brush icon and this window will appear:

You'll be amazed at the number of effects/options available for you when you choose to edit. You will first see six main editing tools: adjustment, effect, overlay, border, sticker and text. 

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