I have a WebView that caches its images into a directory using these settings:
mWebView.getSettings().setCacheMode(WebSettings.LOAD_CACHE_ELSE_NETWORK);
mWeb
The resources are saved inside ApplicationCache.db which is an SQLite database. The images you're looking for could be retrieved from the database, but the format of the database may change between Android versions so implementing a compatible way to do this will be hard. I think you'd be better off downloading and caching the images yourself, outside of the WebView.