The image loader class would append different domains (round-robin style) to each image calls to ensure JS code is loading resources from a spread of domains (to get around the 2 downloads per domain browser limit)
A convinence class could be used to allow the developer register the various image domains (which by default would be set to none). Of course the developer would need to ensure they deploy all images to various domains and setup DNS etc.
First, I assume you mean Ext GWT, not GWT-EXT . Second, almost all images are defined in CSS, which is currently needed to easily support themes and provide separation of styles from code.