
Cut out your image. I like to use the polygonal lasso tool. It's tedious but it does a thorough job.
Unlock the background layer
Got to SELECT>INVERSE
EDIT>CUT to cut out the background. This should leave a transparent background.
Go to SELECT>DESELECT
Next make a new document by FILE>NEW>NEW FILE. I make mine 10x10" and white.
With the move tool drag your image into the 10x10" document.
Resize the image (if necessary) to occupy only the top half of the 10x10" document and center it vertically.
Duplicate the image and flip it vertically.
Line up the images until you have (in this case) 2 girls joined at the feet.

Merge down to join the 2 images into one.
Duplicate several times and rotate each pair until it resembles the picture above.

To make a picture like the one above:
Begin with the resized and centered image.
Duplicate it and flip it horizontally.
You will have a set of mirrored images.
Merge down to combine as one.
Duplicate and flip vertically to have opposing images.
Merge down.
Follow the instructions above.

To make a picture like this one:
Take the resized and centered image, duplicate, flip horizontally.
With the move tool, tilt each image towards each other.
Follow the above directions.
Tips:
This tutorial assumes you know basic Photoshop. It is easy to learn from videos on YouTube or instructional web sites. Google "Photoshop Tutorials" and be amazed at the vast number of hits.
This is done with basic tools available with
GIMP or
PICNIK, both free downloads that have many of the same properties as Photoshop.