There are many websites that allow you to download blog backgrounds or you can use an image that is already saved on your computer. Photo hosting websites such as Picasa, Flickr and Photobucket allow you to upload images for free. Your blog hosting service may also allow you to upload images in a similar way to how the rest of your blog is hosted. To do this you need to open the image in your Internet browser and copy the URL where it is displayed. Here is the HTML code.   body { background-image: url(image URL);  You need to add the code where the body of your blog HTML begins. Replace "image URL" with the full URL where your image is located. You should see the image added as the background.

Summary:
Select the image that you want to add as the background. Upload the image. Get the image URL. Insert the code for adding an image as the background. Save the changes after you edit HTML and view your blog.