So, I am not sure specifically how to change the logon screen user account image as you have requested in this specific circumstance. I ran into a similar issue, but mine was fixed by just picking a custom image from the Change Account Image selection, then Browse. After reading some of the posts, here are my thoughts:
For one, a good resource editor can be found for free called ResHack (Resource Hacker) and the link to the homepage for it is on the Wiki page for it, or can be found here:
http://angusj.com/resourcehacker/
Secondly, since the fingerprint scanner is typically used for primary or supplemental login to the machine, it would have to load a driver hook for the logon mechanism. As for Windows Vista/7 specifics on this, I cannot say for sure. On Windows 2000/XP there was a file called MSGINA.DLL which could be hooked to create this sort of effect. It was commonly used for Netware logins.
The reason I bring it up is that when you modify the login mechanism in this manner, it must load 3rd party code into the login code space to provide the extended or overwritten functionality. In such a case, anything that the DLL wants to load along with that code, is up to the writers of the 3rd party DLL. It sounds like, in this case, Dell decided to have it put their logo as the logon account picture by means of the hook DLL for the logon mechanism.
While I do understand that you would really prefer a method of just click/select to change the image, it likely will not be quite that easy. So, a couple solutions were suggested which would permit you to change the image yourself, by force. Changing the image in the DLL file should do the trick. You will have to locate which specific DLL it is for that (which I believe someone already did for you), but sadly, I doubt there is a more simple way to do it.
There is also another resource file editor that is free with Visual Studio (which is not in the free version of VS if I recall), called ResEdit (Resource Editor). It is a bit trickier to use, and I would suggest a hex editor or 3rd party resource editor like ResHack. ResHack is pretty much just a drag/drop operation, then save the file and reboot. I do not believe it gets any easier than that.
Sorry to hear about your troubles. Messing with the login mechanism for Windows can be a bit tricky. Any other methods I can think of off the top of my head are much more complex and likely unwarranted.