Get the picture you like as big as you can on the screen or if it is in a window, try and make it fill the entire window. Use "print screen" (to take a snap shot of the whole desktop) or "Alt + Print Screen" (for the active window). Open up "Paint" and use "Ctrl + V" to paste the picture you have just taken into it. You can use the cropping tools to select the bit you like then save it as a .bmp file in a location you can find. You will then be able to use this as the wallpaper (your desktop background).