Attached Link: http://www.sparkyworld.co.uk/3d7own.htm

*Stonemason's ruins? *already In the list above :) 

*everything is covered with a light dusting.. *what a beautifully grim description. 
Sounds like a day trip to London :) 

some cars or vehicles, 
Kalebdaark and Helgard have nice ranges of post-apocalyptic vehicles. 
Kalebdaark is in store here - Helgard at Vanishing Point. 

A quick way of making busted cars is download some from here... 
http://dmi.chez-alice.fr/models1b.html ...
remove parts like doors tires etc etc, 

Do a quick remap (box) in UVmapper on the body. On the body map use the seamless tile rust texture in Eye Candy (Nature I think) tweak it to give a burnt feel. 

Flood fill a 512x512 with a slighlty darker version of that texture and use that on all other car parts. Thats how I rust up crashed helicopters. 

skeletons 
For a cool image stick a skeleton in a space suit minus the gloves and helmet. Make the head invisible and use 1 of Poisen's (in store here) skulls. 

slight top down view and/or lack of detailed background 
Thats easy to fix and it also gets around the problem of system resources. Just render the background scene seperately. Or use a vue render. Use that as a background image in Poser or on a background prop like the cyclorama or infinity cove. 

Using a ground prop like the infinity cove ground, this is where you need to use the highest res map. Add some debris - maybe a few buildings in the back. Use the same light set from the background image. Add figures and render.Job done. 

As mentioned above the free DSU prop for poser has one of these images. 

If you are animating and want the character to walk behind stuff and don't need to move the camera - try the tutorial on Alpha Planes in Animation (link above post). 

*sure it's a smaller niche *Yep. Real buildings sell better.

Pinky - you left the lens cap of your mind on again.