Windows 7 lets you easily change the image you see on your desktop. This is called the Desktop background, or Desktop wallpaper.
To chage your Desktop Wallpaper, click anywhere on the desktop with your right mouse button, and you should see a menu appear:

One of the items on the menu allows you to personalize your desktop screen. So click on Personalize with your left mouse button to see a window appear. This one:

The main area in the screen above let's you select a Theme from the available list. A Theme sets not only your desktop background, but also a screensaver, the color of windows, sounds, and mouse pointers. And all that with just one click!
If you just want to set background wallpaper, however, select Desktop Background from the bottom left. The default is called Harmony.
When you click on Desktop Background you should see something like the following:

You can select a picture from the ones available by clicking on it with your left mouse button. Your desktop background will then change. To make the change permanent, click the Save Changes button at the bottom.
If you have a picture of your
own, you can use this instead of the ones built-in to Windows 7. To set your own
picture, click the dropdown list at the top, where it says Windows Desktop
Background:

Here, we have moved to the Pictures Library folder to see what images are available. Another option is to click the Browse button. When you do, you'll see a dialogue box appear:

Browse to the folder where your images are and click OK. We have a folder called York with some images in it, so we've selected that one.
When you click OK, you'll see the images in that folder:

Select the image you want to appear on your desktop from those available. If you would like a slideshow of all the images in the folder, then click the Select All button. If you click the Select All button then the dropdown list at the bottom becomes available:

This list lets you choose how often each image in the slideshow is displayed before moving on to the next one. The default is 10 seconds.
Another area to play around
with is the Picture Position. Click the black arrow to see a dropdown list:

Select each one in turn and watch what happens.
When you are happy with your
choices, click Save Changes. You will then be taken back to the first screen:

Another interesting area to
play around is the Windows Colour area, circled in red in the image above. Click
the link to see the following screen:

Things like the start menu,
the taskbar, and the areas around programs can all have the same color scheme.
The default is called Sky. Click one of the other colors above and watch what
happens to your Taskbar at the bottom. Click the round Start button on the left
of the Taskbar and you'll see that its color too has changed. Uncheck the box
that says "Enable transparency" and you'll see a deeper color for the one you
have chosen.
If you don't like the colors
available, click the arrow to the left of Show Color Mixer. Then play around
with the controls. To get back to the default colors, just click the color
square in the top left of the screen, which is Sky.
In the next section, we'll have a closer look at the Taskbar in Windows 7.