Since NagVis 1.x you can add "stateless Icons" to your maps, we call them shapes. These icons are completely like the other icons, they just have no state.
A shape is nothing else than a simple image. We recommend to use PNG as format for the shapes.
Almost every graphic art software has the ability to build PNG-files.
The shapes are placed in
nagvis/images/shapes. Just copy them to this directory, then you can use them in your maps.
Open one of your maps and add a new shape-icon. You should now be able to select your own shapes.