For an ArcGISDynamicMapService layer, the legend symbols are coming from the renderers defined in ArcMap (either by the new 10SP1 legend service or by the arcgis.com legend service).
So, the legend in the web client is supposed to be very close of the symbols you see in the ArcMap TOC.
If, for any reason, you want to get the images in your client application, you can access to the legend items tree by using, as starting point, the 'LayerItems' property of the legend control. From a layerItem, you can access the collection of LegendItems which gives you the ImageSource (and the Label) of the legend item.