Sounds very simple but I know people out there that actually can not do this and I thought maybe some people who are on the internet cant do this.
This is a very easy to follow guide designed for new comers to computers to allow them to follow step by step and change there desktop back ground.
First on your desktop right click, on the menu that appears select options this should bring up this option box

I have drawn arrows to all the major parts, first the top arrow, you need to be on the desktop tab to change your desktop, left click on this tab to bring up the desktop options.
The next arrow down points to all the different backgrounds that you can have on your computer, browse the list and left click on one you want to see, it will change the image on the computer monitor diagram to allow you to see what it looks like.
The browse button will allow you to browse your computer and load an image that you have stored on the computer this could be a photograph of a love one.
The position will determine where the image is placed, if its centred, stretched or tiled these explain them selves by their names.
Colour will set the desktop colour, if you have an image centred and it doesn’t fill the screen the desktop colour will fill in the rest.
Once you have chosen what you would like click, apply then ok and your done you have changed your desktop background!