There is still no webapp folder. Quoting Leif Åstrand in a blogpost about [static resources in Vaadin Flow]
(https://vaadin.com/blog/vaadin-10-and-static-resources):
such files should be put in src/main/webapp/frontend/ … and … Since the beginning of Maven, files from src/main/webapp/ have been packaged into the root of the .war file.
In my project, I have a logo stored in src/main/webapp/frontend/img/logo.png
And I show it successfully using this:
Image logo = new Image("frontend/img/logo.png", "logo");