Here’s a quick easy way to modify the icons SharePoint displays for different file types. This procedure can also be used to configure an icon for a custom file type you may have.
Be sure to back up any files before making any changes!
1. Open the 12 Hive (C:\Program Files\Common Files\Microsoft Shared\web server extensions\12)
2. Browse to the ...\TEMPLATE\XML folder
3. Open the DOCICON.XML file
4. Modify an existing record, or add a new one.
Here’s an example of an XML file that a customer of mine was working with and the corresponding entry I added:
Invoice.cif
<Mapping Key="cif" Value="iccif.gif" EditText="Customer Invoice File" OpenControl=""/>
The Value property of the node points to the /_layouts/images virtual path which can be found in the 12 Hive in …\TEMPLATE\IMAGES - put your (.gif) icon image there.