IP Based Image

[eluser]Unknown[/eluser]
Hi, i have a banner area on my website where i would like to display an image. I would like to display a different image in the banner area, depending on a users' location.

I have 4 different images:

01 - usa_banner.jpg (default)
02 - europe_banner.jpg
03 - uk_banner.jpg
04 - australia_banner.jpg

If a user views the site from a country outside of the top 4 areas, then i would like to display the default usa_banner.jpg image.

I have created my site using codeigniter. What is the best way to work out the IP ranges etc. Any advice on this would be greatly appreciated. Thanks in advance...