I was just viewing all vista dll files in PE Explorer, when I discovered this.

First, what u should have.
(i) A resource editing tools (like PE Explorer)
(ii) Ur favorite picture in the following resolutions:
(a)1280 X 960
(b)1024 X 768
©1600 X 1200
(d)1440 X 900
(e)1920 X 1200
(f)1280 X 768
(g)1360 X 768
(h)1024 X 1280
(i)960 X 1280
(j)900 X 1440
(k)768 X 1280
(l)768 X 1360
One thing if u r going to use it in ur 4:3 monitor then u just have to ready only b & c.

To change the background picture of login screen open imagers.dll (all dlls r located in system32). Now expand image and replace corresponding picture of that resolution. You can see the resolution in this way.

 Reduced: 73% of original size [ 786 x 561 ] - Click to view full image

Now the default text color in login screen in white. For my case the texts were not visible. So if u want to change the color of the text open authui.dll and goto UIFILE -> 11000. Follow the screen shot. U can also use argb instead of "black", "white" etc...

 Reduced: 73% of original size [ 786 x 561 ] - Click to view full image

The normal logo is displayed in white. If u want to make it black. open the file X:\Windows\Branding\Basebrd\basebrd.dll. And goto Bitmap -> 120. And edit the picture just like me.

Reduced 62% 786 x 561 (60.79K)

I also want to change the Logo text, but I can't. If anyone can help, Please let me know how to edit it. That's all. If I can find anything else on this, I'll upgrade the post.
Happy, hacking.....