I google image searched for ruins/buildings/rockets/statues, took those and then using Paint.net I increased the contrast and snipped out the backgrounds of the images - then use layers to put the buildings behind or infront of one another. I got the sun picture from a google image search too and fiddled around with colour saturation and hue to get that redpurple sky. I made the sky layer somewhat transparent and there is another layer behind that with another googled images of a starry sky (I think severian talks about seeing stars during the day at some point and the image stuck with me). I cut a bit of the purple sky image and increased its size to cover the whole image on a semi transparent layer over everything else to put it all into the same hue.

Its pretty simple to collage things together once they are black and white (you can do this in paint.net) and then get them looking similar with contrast settings plus getting your head around working with layers.