After intense testing I can say it does work with background images and ACF.
So in case of ACF check that return type is ID!
thanks for the great plugin!
Ok Stefan,
I think I got it. ACF returned the images as image-urls instead of image id because my theme set the fields to return image urls. after switching the ACF fields to return image ids it seems to work.
I still have some problems but might be able to fix them on my own.
