Just a notice guys; you can't just "Google" what you want and slap it into the background for a few reasons:
1. Most of the time, it is against the law. Those are most likely peoples' images and have some kind of copyright on it. If you upload a photograph to the internet, even if you haven't watermarked it or anything, no one can use it without your consent. Try to contact the owner of the photograph, take your own, shoot on location, or find a royalty stock library (that is, if you don't want to pay too much for it.)
2. It is going to look terrible if it is just stationary, no grass moving, no foreground elements, stationary camera, etc. It's boring, and usually inrealistic. Of course, I'm just making assumptions, but using a landscape like this
will look bad and I am sure you can find a similar location. You'd have to do some kind of CG sky (or stock footage of moving sky) add a lot of camera tracking info to distract the viewer from the obvious flaws that a video has when using still as a background, etc.
3. For something like landscapes, try to shoot on location. I know this is the backdrop forum (I provided info prior to this), but landscapes go best on location. If you need to, get a really basic small outdoor sets that look like a forest in the frame and improvise. It will always look better than keying. If you don't have the location resources at all (maybe you live in the desert and want European landscapes of WWII), then I guess you'll just have to use your imagination in the compositing process. Here are some good resources:http://istockphoto.comhttp://fxhome.com/forums/viewtopic.php?p=232851#232851
I am serious, if you don't have about 10 things going on, weather it's explosions, fog, dust, background environmental elements, background things, artificial camera movement (tracked from a camera maybe?), with a combination of creative angles, editing, and keying, it will look terrible. I wish you luck.