Use a Shape for Layers, Image for Styles

Export Kit will render all your PSD layers as-is. So this means that if you have an image layer in your layout – it will render as an image.

CSS Styles are dynamic and can apply to any layer.  Because you are using CSS Styles with Images, this means that you are also using CSS Images (CSS Images use a DIV tag).  

Because shapes also use a DIV tag, and DO NOT render images – you can use a shape rather than an image in your design – and add the image to the CSS style.

Using shapes in your layout will render only the images in the styles, and prevent ”extra image assets” in the skins.