1. Choose two pictures, here one of Rose and of Jack, and crop them into squares that are 100x100 pixels. (tip: to make an exact square with Photoshop, hold down the shift key while you make a selection)

2. Using either picture, go to Image | Canvas Size and click the top center box, so the added space only goes down, and increase the Canvas Size by 200% the height.

3. Drag or cut and paste the second picture onto the first picture so that it covers the lower half.

4. Rotate the canvas (Image | Rotate Canvas | Arbitrary) 45 degrees CW.

5. Open a new document that is 200 x 200. Copy Merged and paste the picture of Jack and Rose onto this new image.

6. Offset the layer of Jack and Rose so that a diagonal is created directly across the image. Copy, just the Jack layer, from the Jack and Rose image, onto the new document. Offset it so it continues the diagonal stripe.

7. Change the Image Size on the Jack and Rose image to 50 percent. (In the example they are vertically flipped.) Copy and paste them over to the new document and offset them just like the larger Jacks and Roses so they fill in the gaps between.

You may want to make some darker or lighter, which would mean putting an adjustment layer (Layer | New | Adjustment Layer) between them. Choose Levels and play with the settings until you like what you see.