From http://openlayers.org/dev/examples/image-layer.html: The "City Lights" layer below is created from a single web accessible image. If you construct it without any resolution related options, the layer will be given a single resolution based on the extent/size. Otherwise, it behaves much like a regular layer. This is primarily intended to be used in an overview map - where another layer type might not make a good overview.

The image layer is defined by type|url|title|size.

[map layers="1" 1="image|http://earthtrends.wri.org/images/maps/4_m_citylights_lg.gif|City lights|580,288" maxextent="-180,-88.759,180,88.759" width="580" height="288" controls="statusbar"/]