Relative image location question

I have two Vaadin applications.
The following CSS is as seen by Firefox inspector.
App number 1 seems to accept background-image: url(“img/someimage.png”)
App number 2 does not properly resolve images as above. Instead, it requires a longer string: url(“./VAADIN/theme/mytheme/img/someimage.png”).

What is the difference between app 1 and 2?
In other words I’d like to use the simpler version in App 2.