How to add image in Custom login form

Can I possible to add images in my custom LoginForm that extend vaadin LoginForm

Technically yes, if you override the getLoginHTML method. But if I were you I’d probably use a CustomLayout instead and handle the textfields myself.

where I put that image to access

There are several places you can put it; If you use a CustomLayout, then in the same folder as the layout. If using ThemeResource, then in you theme folder. Other options still are using FileResource, that can access any location on the server hard drive, or ExternalResource, that can access any file on the Internet using an URL.