How to Make a Dark, Post-Apocalyptic City Illustration

In this tutorial, we'll change a full of sunshine, ordinary photograph into a gloomy image of the world after destruction of mankind. Using simple tools, we'll turn lively streets into abandoned ruins overgrown with weeds. A number of stock images and a few little tricks will let us optimize our work and make this job more interesting and spectacular. So let's go!
Take a look at the image we'll be creating. You can view the final image preview below or view a larger version here.

All the photos I've used are taken from the stock.xchng site. Let's take a look on them, before we start: the city, the flag, the texture 1, the texture 2, the texture 3, the grass texture, the can, the can 2, the banana skin, the old newspaper, the grass, the bushes, the bushes 2, the plants and the clouds. Also I've used one set of brushes, found on DeviantArtBirds Brushes.
Open the city image, which you intend to use, and save it under a different name. The first thing to do in order to achieve the appearance of the deserted, empty city, is to remove all signs of life, like people and lights. Use the Clone Stamp Tool and Brush Tool, which we'll be useful for finishing the details. Both tools should be soft-edged and have a size of 5 to 10 pixels.
Now, in order to create the appearance of destruction, several actions need to be taken. First, change the appearance of the dark, blue banner (center-left of the image). Use the Brush Tool to erase most of the gold letters, and use the Clone Stamp Tool to remove the lower part of the banner.
Open the flag image, and separate the flag from the background. Go to Select > Color Range to select all the blue pixels and press Delete. Use the parameters as in the image below.
Select the Eraser Tool (large size and hardness set at 0%) and remove the side part of the flag.
Finally, make the image monochromatic (Image > Adjustments > Black & White).
Place the flag onto the surface of the main image. Use the Move Tool to move the layer into the appropriate position and the Eraser Tool to remove the unnecessary pixels.
Then go to Image > Adjustments > Levels and set up as shown below.
Use the following tools to finish the work here: Clone Stamp Tool, Eraser Tool, Brush Tool, Burn Tool, and Dodge Tool.
Also select the Brush Tool and treat all other banners and flags in a similar way.
Transform the windows now. Use the Brush Tool to remove some of the glass and alter the details. Apply some transformations with the Polygonal Lasso Tool (L) and Move Tool if you want to.
Review the whole main image looking for areas where changes need to be made. When transforming the details, first mark them using the Polygonal Lasso tool, and subsequently cut out, paste in as new layers and transform them with the Move Tool. Use the Brush Tool during the process, if necessary.

The last stage of creating an impression of destruction is removing flowers from the street. Paint all the flowers over with black, using the Brush Tool. Make up the missing parts of the flower pots with the Clone Stamp Tool, and add the shadows using the Burn Tool.
Now we need to work on the textures. Create a new layer and go to Filter > Vanishing Point to apply the texture 1 onto the surface of asphalt. If you don't know how to use this filter, I recommend you read Alvaro's tutorial Getting to Grips with the Vanishing Point Filter before starting this step.
Change the Blending Mode of the layer to Multiply. Select the Eraser Tool and remove the unnecessary parts of the texture.
Finally place texture 2 onto the sidewalks in the same way.
Overlay texture 3 over the buildings, in the way similar to the sidewalks and street, but do not use the Vanishing Point filter.
The last texture to be overlaid on the street surface is the grass texture. Contrary to the previous ones, in this case set the Blending Mode to Overlay.
Now we will use a large number of stock photos. They will help make our image more detailed and interesting. Place the images of the can, the can 2, the banana skin, the old newspaper, the grass, the bushes, the bushes 2 and the plants in the document, resize them and erase the backgrounds.
Select the Burn Tool and apply some shadows.
Next select the Brush Tool, change its Blending Mode to Color. Alter and adjust the new elements so that they coexist better with the texture and color of the main image.
With a Brush Tool already selected, change its Blending Mode back to Normal and add the settings in the Brushes Panel as shown below.
Paint some blades of grass, trying to make this as realistic as possible.
Go to Image > Adjustments > Hue/Saturation using the settings below.
At the end, use the Brush Tool, Burn Tool and Blur Tool in order to add more flora, and make the image more appealing and consistent.
Select the Burn Tool and add some hard shadows on all the walls.
Use the Brush Tool with the Blending Mode set to Color to add some
green smudges.

Change the Blending Mode of the Brush Tool back to Normal once again, and add the settings in the Brushes Panel, as shown below.
Using a green color for the brush, add some flora on all the buildings.
Use the Burn Tool and Dodge Tools to make highlights and shadows. This will enhance the effect of three-dimensionality.
Add some green and flora on cars, boards and flower pots using the same methods. Look at the third image to see all the successive small steps of working on the white road sign.
Import the photo of the clouds into the document and place it in the bottom part of the image.
Next add a Mask to the Layer. Use the Pen Tool or the Magic Wand Tool to make a selection on the sky, next go Select > Inverse and Fill the selected area (of a mask, not an image) with black (using the Paint Bucket Tool).
Go to Image > Adjustments > Black & White and use the default settings.
Use the Burn Tool and the Dodge Tool to change the contrast of the clouds, if you want to. Finally, select Brush Tool to paint the clouds with a new hue. First set the brush to a big size (about 150 pixels), hardness set to 35%, and Blending Mode set to Color, and use some pastel shades.

Without changing any of the brush settings, add some warm colors to the buildings and streets. At this point, your image should look like the one below.
Download the Birds Brushes set and open them in Photoshop. Add a few birds in the sky using a proper size.
Now it's time to add final effects to your image. At this moment you must have only one layer. Simply save your work as a JPG or go to Layer > Flatten Image to do that. Next go to Image > Adjustments > Levels and add the settings represented in the image below.
Last but not least go to Filter > Render > Lighting Effects and set up as shown.
Well done! You've now seen the whole process of turning an image of a lively city into a state of decaying ruins. Let us hope this will never happen in reality. You can view the final image below or view a larger version here.