Question

if webview html content has few <img> tags, is it possible to opens that image in fullscreen mode in new activity when user hit image in webview?

Answer 1

Yes it is. basically it is possible to invoke any kind of native Android code from within a WebView. You define the methods you want to expose for calling from within the WebView in a class, pass an instance of this class to the WebView and from then on the methods are invocable just like javascript methods. here comes the example (this is copy-pasted form another answer of mine; you will need to replace the video intent with image displaying intent):

Calling native code from within web view:
When creating the web view add javascript interface (basically java class whose methods will be exposed to be called via javascript in the web view).

JavaScriptInterface jsInterface = new JavaScriptInterface(this);
webView.getSettings().setJavaScriptEnabled(true);
webView.addJavascriptInterface(jsInterface, "JSInterface");

The definition of the javascript interface class itself (this is examplary class I took from another answer of mine and opens video in native intent)

public class JavaScriptInterface {
    private Activity activity;

    public JavaScriptInterface(Activity activiy) {
        this.activity = activiy;
    }

    public void startVideo(String videoAddress){
        Intent intent = new Intent(Intent.ACTION_VIEW);
        intent.setDataAndType(Uri.parse(videoAddress), "video/3gpp"); // The Mime type can actually be determined from the file
        activity.startActivity(intent);
    }
}

Now if yo want to call this code form the html of the page you provide the following method:

<script>
  function playVideo(video){
    window.JSInterface.startVideo(video);
  }
</script>

So you need to add the appropriate method to JSInterface and call the code from within the Web App.