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:

You can see that, particularly for labels, they are duplicated. This behavior is quite simple: each tile is requested separately in a tiled WMS. Most of the time the position of the label depends on the position of the object, but here you can request it more than one time because of your tiles. In conclusion, be careful to not use tiled WMS when you are using automatic labeling in WMS or you will suffer from labels' duplications.
After this functional review, let's inspect the WMS layer ImageWMS source class properties:
The constructor is ol.source.ImageWMS. Contrary to the tiled version, you can't call multiple URLs. The options are available as follows:
After this raster overview, it's time to review a bit more on how to combine those different layers in a big mashup. We will illustrate it combining the layers based on OpenStreetMap projection, the Spherical Mercator.