By default, you can't easily change the Windows 7 logon screen background to an image of your choice. But as usual, some clever people have put together some tools to allow this with just a few clicks.

The following tools are both popular, free ways of doing this:

Julien Manici's Windows 7 Logon Background Changer

Tweaks.com Logon Changer for Windows 7

Manual Method

If you want to manually change the login background, you can do so using these instructions. This requires some understand of registry editing, so should only be performed if you understand the consequences. These instructions are for advanced users only, and the above tools should be used if you are unsure.

First, create a key named OEMBackground in HKLM\Software\Microsoft\Windows\CurrentVersion\Authentication\LogonUI\Background if it doesn't already exist and set it's value to 1.

You can now create/replace images in the following folder with your own choice of logon background:

%windir%\system32\oobe\info\backgrounds

These images need to be under 250k in jpg format with the following filenames (sorted by aspect ratio):

backgroundDefault.jpg
background768x1280.jpg (0.6)
background900x1440.jpg (0.625)
background960x1280.jpg (0.75)
background1024x1280.jpg (0.8)
background1280x1024.jpg (1.25)
background1024x768.jpg (1.33-)
background1280x960.jpg (1.33-)
background1600x1200.jpg (1.33-)
background1440x900.jpg (1.6)
background1920x1200.jpg (1.6)
background1280x768.jpg (1.66-)
background1360x768.jpg (1.770833-)

If your resolution isn't listed in one of the above file formats, the backgroundDefault.jpg will be stretched to fit.