Electron and assets

I am using ionic 2 to create a desktop app. Everything works fine. I have a issue trying to access assets. For my assets I have :

Failed to load resource: net::ERR_FILE_NOT_FOUND

Those errors concerns for example url in css :

page-dashboard .main-back{
	background-image: url('/assets/img/sheepsatmyvatn.jpg');

I cannot find on the web, clear solution on how to fix this problem. Any clues ?