I use html-like labels with a picture. For example, a picture of size 24x24 is placed on the edge with the following label:
label=<<TABLE BORDER="0" CELLBORDER="0" CELLSPACING="0" FIXEDSIZE="True" WIDTH="12" HEIGHT="12"><TR><TD><IMG SRC="img.png" /></TD></TR></TABLE>>
When generating, warnings are displayed about insufficient space for the label, but everything is drawn correctly. Additionally, you can add a text label via 'xlabel'.
