Lazyload placeholder

I am trying to add a placeholder to a lazyloaded background image.

Reading the docs it stated I should be able to extend the app property to include ‘lazy’ and set the placeholder string as such

lazy:{
   placeholder:"img/logo.png",
},

But it never seems to load the placeholder? I have tried setting the path as

placeholder:“img/logo.png”,
placeholder:"/img/logo.png",
placeholder:"…/img/logo.png",

Still nothing, I don’t get any errors and the images load in ‘lazy’ so I know the lazy is init, not sure what I’m doing wrong here.

Thanks.