First let's have a look at the width and height of your screen.
There are 2 main groups of computer screen resolutions, the 4/3 group and the 16/10 group. Besides these, there are a few other groups like the 5/4, the 16/9 and the 11/6.
Wallpaper for 4/3 screens
The 4/3 group of resolutions are the resolutions with a width equal to 1.33 x the height. They are descending from the original VGA resolution of 640 x 480 pixels. The figure above is a 1/5 scale image with all proportions kept.
Wallpaper for 16/10 screens
The 16/10 group of resolutions are the so called wide screen resolutions, which are common on laptops and now also on desktop screens. Their width is equal to 1.6 x the height. The figure above is a 1/5 scale image with all proportions kept.
Wallpaper for 5/4 screens
The 5/4 group consists essentially of one screen resolution, the 1280 x 1024 pixel screens. The width is equal to 1.25 x the height. It is found on classical 17" and 19" LCD flat screens.
Choice of the wallpaper resolution
The best advice is to take always a wallpaper that has the exact resolution of your screen. You can also take a wallpaper of the same group, let's say a 800 x 600 wallpaper on a 1024 x 768 screen, both sizes being in the 4/3 group. If your computer is set to stretch the wallpaper, the 800 x 600 wallpaper will fill the 1024 x 768 screen without distortion since both sizes do have the same proportions of 4/3. But the sharpness of the image will suffer a little bit.
However, if you take a wallpaper of another group, i.e. of other proportions, it will either not fill your screen or it will be seriously distorted.
Wallpaper resolutions in the 4/3 group
Resolution
Proportions
Name
640 x 480
4:3
VGA
800 x 600
4:3
SVGA
1024 x 768
4:3
XGA
1400 x 1050
4:3
SXGA+
1600 x 1200
4:3
UXGA
Wallpaper resolutions in the 16/10 group
Resolution
Proportions
Name
1280 x 800
16:10
WXGA1
1440 x 900
16:10
WSXGA or WXGA+
1680 x 1050
16:10
WSXGA+
1920 x 1200
16:10
WUXGA
Wallpaper resolutions in the 16/9 group
Resolution
Proportions
Name
1366 x 768
16:9
WXGA2
480 x 272
16:9
PSP
1024 x 786 Wallpapers
Many people are searching for 1024 x 786 wallpapers. This is obviously a typo. They are probably looking for 1024 x 768 wallpapers.
1400 x 900 Wallpapers
Some people are searching for 1400 x 900 wallpapers. This is obviously a mismatch between the resolution of 1400 x 1050, which is a 4/3 format and the resolution of 1440 x 900, which is a 16/10 format. 1400 x 900 is not a current wallpaper format.