VertexRenderingFunction is an option for GraphPlot and related functions which gives a function to generate the graphics primitives to use in rendering each vertex.
VertexRenderingFunction->f specifies that each vertex should be rendered with the graphics primitives given by f[rk, vk], where rk is the coordinate position where the vertex is being placed, and vk is its name.
The graphics primitives obtained by applying VertexRenderingFunction are in effect directly spliced into the graphic generated by a function like GraphPlot. Inset can be used to insert an arbitrary expression.