That is relatively easy. You need a picture with 2 unfilled areas. It must not be content, but must be transparent like a window. Then you take your two pictures and put them next to each other and put the created image just over it. The image must be saved as .png, otherwise surfaces without content will be filled with white. The effect that it looks inclined is due only to the bevel of the stencil over the two images. That's all.