Well, I have a question and didn't know where to go to ask.  Newbie, here.  
I saw Renderosity has backgrounds for sale.  It says 1200 X 900 pixels, with 300 dpi.
Please look at "Old Castles" background or the "Moon" backgrounds.

1.  Are these just pictures?  Or are they partly 3D?
2.  I'm assuming the pixel size then fills the whole screen -- but does it download that size?
3.  How do they create these?
4.  How is it used?  Is it like flat at the back of the scene. 
Thanks
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