Cool backgrounds for GIMP

I have always wanted to know where/how people got/made the neat backgrounds for their pictures. Like if they have a Friesien they have a dark/gothic scene, or if they have a grey arab they have an edited background. I would just like advice.
Like these types of pictures
http://fc01.deviantart.net/fs71/f/20...tar_Manips.png
http://th06.deviantart.net/fs18/150/...by_babekha.jpg
http://farm2.static.flickr.com/1251/...05d515996c.jpg
http://fc03.deviantart.net/fs42/f/20..._amesssx33.jpg
http://fc06.deviantart.net/fs27/f/20...wnTalisman.jpg
*Not my pics, I am just using them as examples of what I mean*
Like how do you make the pictures just flow together

Well I think alot of people use stock images for both horse, background and sometimes other objects in the scene. I've never actually tried it myself though, so im not too sure on all the details for it.

You have to get mutliple pictures, play with overlay, colors, tones, size, opacity, everything until they all match up and look right.

There is no right or wrong way to do it, everyone has their own way, as long as it turns out in the end :)