What do you mean with "good quality"?
On a computer screen, I've never needed the 3000x2000 pixel resolution a camera image has. Everybody who makes pictures meant to be viewed on a screen (that includes all pics shown on websites) knows to make the quality "just good enough" which makes the file smaller.
For example, Irfanview (a well known picture viewing and editing program) has a nice add-on called "safe for web". That makes the picture even smaller while preserving the visual quality.
Kees
Our mother nature is indeed amazing, natural landscape can be quiet, magnificent or inspiring. Personally, I love to have nature landscape pictures as computer wallpaper, and to change it brings me a fresh new mood. However, it is sometimes hard to find a suitable pictures site, wondering if anyone can recommend me such websites, it has to enable free download without registration, and make sure the photos are in good quality.

Chowhound
Comic Vine
GameFAQs
GameSpot
Giant Bomb
TechRepublic