I have come across some tools in changing your Windows 7 Logon Background, and there are some tools i found. this is very cool portable app you can download and i wanted to share this to all of you guys.

7LogonChanger

New Features: v1.2
One of the main feature is it supports Flickr and Picasa Images, with options to search by tag or featured (Picasa) or Interestingness (Flickr).
Another feature is the downloaded Image can be set as wallpaper.

Change Log:
Download Wallpaper/Image/Photo from Picasa or Flickr: Wallpaper selection is based on the tags or Featured/Interestingness.
Auto image conversion:- converts the high resolution images (tested up to 3.77 Mb size and 2560 x 1600 resolution image) local images and web images both.
Set the logon screen back to default.
Now even more easier to set the local images, double click on the preview box, select image, right click and Set as Logon. That’s it.
Download the web images(click on Get Images) select the thumbnail to get the image preview, right click to Set as Logon or Set as Wallpaper.

Download from Here!