I noticed that when I used the Ext javascript library, that an image s.gif was loaded from extjs.com
It turns out the folks from Ext.js have defined a variable which defaults to http://extjs.com/s.gif but you should set it to your local path, as follows:
Ext.BLANK_IMAGE_URL = 'ext/resources/images/default/s.gif';
If you are using Ext 1.1 the image will be located here:
Ext.BLANK_IMAGE_URL = 'ext/resources/images/aero/s.gif';