ol-ext: Layerswitcher image
A simple layer switcher control with image buttons to select the layers.
The title or name layer's property is used to named the ol.layer in the switcher.
displayInLayerSwitcher layer's property to cause layer to not display in the LayerSwitcher
The baseLayer layer's property manage exclusive visibility on layers (only one base layer is shown at a time).
It use the layer.getPreview function