When you start up your computer, and before you have any programs running, there is (or can be) a picture showing in the background. This picture is called "wallpaper", because it acts as a backdrop for whatever else you might have on the screen. People use it to make a boring screen look interesting. You might say, "I haven't got any picture, all I've got is a plain colored or geometrically patterned screen". Well, that's because no-one's ever installed wallpaper on the PC you're using. We can soon change that!
Here are the instructions to set up wallpaper on a PC running a version of the Windows operating system:
(1) go to a wallpaper gallery on this website, then left-click the wallpaper you like;
(2) when the picture has completely finished downloading, right-click on it with your mouse;
(3) if you're running Internet Explorer choose "set as wallpaper" or "set as background", depending on the version of Internet Explorer you're using;
Your wallpaper should now be set to the picture which you just downloaded! You can download as many wallpaper pictures as you want - when you get tired of one, load another up.
There's one more thing you might want to do with your wallpaper - stretch it to fit your screen. Some of the wallpaper photos on this website measure 1024 x 768 pixels, and some are 1152 x 864. Of course there's no way for me to know what size your screen is, and the size of screens keeps increasing anyway. But it doesn't matter, because Windows can take any size of picture and change its size to properly fit the screen, by adjusting your display properties. It's pretty simple - just minimize all of the windows on your PC, right-click on any empty part of the background, and choose "properties". If you're running Windows 98, click the "display" pull-down menu on the "background" tab, and choose "stretch". If you're using Windows NT, go to the "Plus" tab of the properties window and click the checkbox labelled "stretch desktop wallpaper to fit the screen".
If the wallpaper has jagged edges on your screen, then that might be because your web browser isn't doing a good job of stretching it. Internet Explorer stretches photos very well, but version of FireFox up to at least v3.0.1 do it very poorly. The best solution in this case is to set up the wallpaper using Internet Explorer.
I've only ever used PCs running Windows, so I can't help you if you're running a Macintosh, or some other type of computer. Nevertheless, there probably is a way to set up wallpaper on PCs which aren't running Windows, so check your user manuals or help text.