If you want your icon to always be on top in the scene, then you can use
vtkPixelSpaceCallbackMapper and draw your icon on an SVG/div/whatever element on top of the vtk.js viewport. See the Spheres and Labels example for a demonstration of this.
If you want your icon to be physically in the scene (aka other objects may occlude the icon), then you will need to use an ImageMapper+Actor, then listen on
camera.onModified(callback), and update the orientation of your image based on the camera’s direction of projection, as well as the size of your icon on zoom.
If your icon will always be on top, I would first use
vtkPixelSpaceCallbackMapper, since that is much easier to do than listening to camera.onModified.