When you add a raster image to a map using Data Connect, it appears in a Display Manager layer. You can edit the properties of this layer to add an “opacity” property, and then set that opacity property to a value below 1 to change the transparency of the layer.
To make a raster image transparent
You can use any type of raster, including a WMS image. The image does not need to be transparent or have an opacity setting already.
Save the layer to a .layer file.
<Opacity>0.6</Opacity>
An opacity setting of 1 makes the layer completely opaque. A setting of .1 makes it virtually transparent.
The translucent raster image overlays the opaque layers, and they appear beneath it.