Although there are really less image layers' sources available than a tiled one, they are also useful depending on your goal or your backend.
We will start with the image WMS layer and then cover other types.
Like the tile layer WMS already reviewed, this component is also using the WMS standard to retrieve the map, imayer. The thing that differs here is that you add a layer using an ol.layer.Image constructor instead of the now more usual ol.layer.Tile and, your ol.source is ol.source.ImageWMS instead. Next, you just have to complete the parameters like for a tiled WMS.
Now, let's see why we need to use untiled WMS.
See the two following images coming from the official example when you zoom in. The first one comes from http://openlayers.org/en/v3.0.0/examples/wms-tiled.html. See the following screenshot:
The second image, as shown here, is the untiled one from http://openlayers.org/en/v3.0.0/examples/wms-image.html:
