thanks for your reply.
i am using 3.20 version, which does not support #DebuggerForceStandardAssets: true attribute.
since i have tested with two webviews (one with loadhtml & another loadurl), as one of the webview loads asset picture. so i think file is properly copied to project asset folder.
warwound, i am referencing files as <img src ="file:///android_asset/file.jpg" /> so it is not relative reference.