v.surf.icw: add screenshot to help page

Author: HamishB

src/vector/v.surf.icw/v.surf.icw.html

@@ -56,6 +56,15 @@ <h2>NOTES</h2>
 variable is set, the number of concurrent processes will be set at
 that number of jobs.
 
+<div align="center" style="margin: 10px">
+  <a href="v_surf_icw_salinity.png">
+    <img src="v_surf_icw_salinity.png" width="600" height="620"
+     alt="v.surf.icw example" border="0"></a>
+  <br>
+  <i>ICW interpolation of surface salinity in Chalky and Preservation Inlets,
+     Fiordland, New Zealand</i>
+</div>
+
 
 <h2>EXAMPLE</h2>