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    Step by step tutorial on creating a composite image to fill a canvas using Adobe Photoshop's layer masks and layer effects.
Combining images in Photoshop
Create a seamless background using a second image as fill.
I found the perfect stock image while starting a recent project but as usual it was the wrong proportions.  I needed this vertical image to be horizontal and cropping it was not going to work.  The best solution was to fill in the bottom of the image so that it would have enough length to cover the entire canvas area I was working on.  

Here are my original files: a city street at night with light trails and a plain old snapshot of the sidewalk.  All we really need here is the concrete texture so anything that comes close to matching the pavement in the foreground of the first image should work.
I started by creating the canvas, in this case it was a 11x17" vertical composition so the client can print posters from the final image.  With all three documents open I used the move tool (v) to drag the stock photos into my new document.  I arranged them so that the sidewalk texture was beneath the city photo.
Transform the sidewalk layer's shape (cmd-t).  Right-click anywhere inside the transformation box to bring up the dialogue menu and select 'perspective.'  

Try to visualize the vanishing point and perspective grid and match the boundary box to it while keeping the lower half of the canvas covered.  It doesn't need to be exact, we're just giving the illusion that the sidewalk is receding from the viewer instead of being painted flat onto the canvas.
Now the biggest problem for my eyes has been the color of the sidewalk compared to the orange streetlights in the city photo.  Double click to the right of the sidewalk layer's name in the layers palette to open the layer styles window and click on 'color overlay.'   Set the blend mode to color burn and the opacity somewhere around 50-75%.  Then change the color to a dark brown - something that matches the tone and saturation of the pavement in the city photo.
Now we need to blend the opacity of the top layer, so select the city street layer and click on the 'Add layer mask' button at the bottom of the layers palette.  That creates a blank mask which allows you to paint out transparent areas (with black paint) and paint in opaque areas (anything white in the layer mask thumbnail).
Instead of going right in with a black brush, lets use the gradient tool (set with solid black and white) to create a transparent-to-opaque blend from the bottom edge of the layer up to the bottom of the light trails and the intersection in the background.  

After we have that rough fade in place we can go in with a black brush (set to a medium size soft 20-50% opacity) and paint out even more of the layer.  I also went in with the same brush and white paint (just hit (x) to switch between colors) to bring back some of the sidewalk detail on the far left of the image.
Now we're getting close but the lighting isn't right on the new sidewalk texture.  We can add a gradient to the edges by going back into the layer styles (double click on the sidewalk layer in the layers palette) and choosing gradient overlay.  I set the blend mode to multiply, checked 'reverse', selected radial instead of linear, unchecked 'align with layer' and bumped the scale up to 150%.  

Then I clicked on the gradient image and set the opacity stop on the right to 0% (see above image) and moved the midpoint down to about 35% from the default 50% (see image below).  This knocks out the white in the center of our gradient so we can see the sidewalk through it and widens the transparent center so that the shadows are pushed to the edges of the canvas away from the center.
And that leaves us with our final image without any destructive changes (except the perspective shift when we transformed the sidewalk image).  At this point if the client wants more of the original image to show through we can paint more of it back in the layer mask.  If they want the foreground pavement to be less orange we can go back to our color overlay layer style and change the opacity or hue in moments.
Please feel free to contact me via twitter (@carter_isonhart) or email (cartermke@gmail.com) if you have any questions or I missed something - it's been known to happen occasionally!