Tweaks.com Logon Changer
Full user review
"It does the job easily and simple."
Program doesn't require installation. I set a personalized 600x450 JPG image as my Windows Logon Wallpaper (which is stretch to full screen). The program also indicates where you can find the old backgrounds archived.
Well, I didn't try JPG images bigger than 245 KB. Although, the 245 KB limit is imposed by Microsoft.