Choose the size of wallpaper you require. The options are 800 x 600 or 1024 x 768. Click on one of the options and a bigger image will appear in a new browser window. Move your cursor (pointer) over the big image. Right click your mouse and a 'drop down' menu will appear. Go to 'set as wallpaper' or 'set as background'. Finally left click your mouse. Your new wallpaper will appear on your desktop.
To find out the best size for you, right mouse-click on your desktop, select 'properties,' a pop window will appear; select 'settings' and note the dimensions in the section called 'screen area.'