I actually did this a few months ago. I was getting sick of that teal monstrosity on the user select screen, and decided I was going to get rid of it and replace it with a nice pure black background. After a few days of searching, I finally figured out how to do it on Vista (and, for the record, it looks great).
Warning: if you're not careful (for instance, if you don't actually listen when I say "back the file up"), you might end up screwing up your computer. That's not my problem; you're doing this at your own risk. If you follow my instructions, you shouldn't run into any problems, though. If you don't want to do this manually, but still want to change that image, you may wish to try
Stardock LogonStudio, instead.
You won't need a great many tools to do this, it's actually pretty simple (especially since I've listed the resource resolutions here here):
Windows Vista (any version should work)
Resource Hacker (freeware) or equivalent software
The image you wish to use as the background in JPG/JPEG format (needs to match your screen resolution; I just used a 1440x900 black JPEG)
0. Install Resource Hacker by extracting it to somewhere that you can easily get at it (link above).
1. Navigate to System32 (most likely C:\Windows\System32\) and copy (not move) imageres.dll to somewhere that you can easily get at it.
2. Open the copy (not the original) of imageres.dll in Resource Hacker and open the "IMAGE" resource tree.
3. There should be a number of sub-trees here; each one corresponds to a certain screen resolution. Here's what each one corresponds to:
5031 -> 1280x1024
5032 -> 1280x960
5033 -> 1024x768
5034 -> 1600x1200
5035 -> 1440x900
5036 -> 1920x1200
5037 -> 1280x768
5038 -> 1360x768
5039 -> 1024x1280
5040 -> 960x1280
5041 -> 900x1440
5042 -> 768x1280
5043 -> 768x1360
If your resolution is listed here, replace the resource by opening the appropriate folder, right-clicking the image name (probably 1033), and clicking "Replace Resource" and navigating to your image on the popup. Enter IMAGE for the type, the 50** number for name (e.g., 5035 for 1440x900), and the image number (probably 1033) for the language.
If your resolution isn't listed here, then I'm not sure what you should do. I'd assume that Windows uses one that has an equivalent aspect ratio, so try replacing those.
4. Save the changes and exit Resource Hacker. Rename the imageres.dll in System32 to imageres.dll.old (you have to have "show known file extensions" enabled to do this) and move the new imageres.dll to System32. I believe imageres is only used at logon, so it shouldn't be in use. If it is, you might have to do this step in safe mode.
5. Logoff or switch users or shutdown or whatever and you should see your new image as the background for those screens.
Unfortunately, you can't take screenshots of this screen, so I can't show you the final result. Suffice it to say Vista looks better in black than it does in teal (the transparency effects on the buttons makes it look quite professional).
Update: it appears you may need administrative privileges to edit the file. If so, you can get those by opening an
elevated command prompt and entering the following:
>takeown /f "%SYSTEMROOT%\system32\imageres.dll"
>icacls "%SYSTEMROOT%\system32\imageres.dll" /grant administrators:F