When I first started to customize logon screens in Windows using a resource hacker to replace bitmaps in logon screen DLL files was the only way. This would get the job done but it would also break digital signatures on the system files, something that Microsoft values more and more with each OS release. For the first time in Windows with the Windows 7 release, Microsoft is providing the ability to customize the logon screen background without hacking any system files.
Using a feature that was originally designed for OEM computer manufactures like Dell and others to brand their computers, everyone can now safely customize their Windows 7 logon screen background.
The actual process requires creating some registry keys, folders and files. To simplify the process I created a free utility called the Tweaks.com Logon Changer that will allow you change the logon screen background with just a few clicks.
After you download and start the utility just click Change Logon Screen and then select your new background.
The new background will be active as soon as you select the new background. Click Test to see it in action.
Make sure that the background you select is smaller than 256KB, otherwise it will be rejected because of a size limitation set by Microsoft.
If you want to remove your customized background just click Revert to Default Logon Screen and the registry changes will be removed.
If you would like to manually tweak your logon screen by editing the registry and copying files visit Rafael's tutorial here.