I have chosen 2 different computer case.
Please download the files from the bottom of the page.
Open the white.jpg file (the white case) and with the Polygonal Lasso Tool make a selection like mine
Then press Ctrl+J. You will notice that we have a new layer in the Layer Pallete.(rename it Right)
Open a new document and drag the layer "Right" on this document. Rename this document to "Best.psd"
Then go to Filter > Distort > Shear and make something like me.
Now go back to the first image (the white case) and in the History Panel press on "Delete" until you will see the same selection (you can press also Ctrl+Z to undo the last action). You should have the same image with the selection like on the first step.
When you are on this step go to Select > Inverse.
Then press one more time Ctrl+J.
The same as above a new layer is created.drag this layer on the "Best.psd" and then go to Filter >Distort >Shear and for this side use this settings:
Then you should have something like this.
Now open the blue case and drag the photo on the best.psd
Place the blue case in the middle.Be sure your layer with blue case will be the second layer after the background.
This is a very nice effect.You can try also with 2 bannanas, one skull and a head, 2 bottles , 2 monitors.






More Photoshop: