Ian Davis created a number of FOAF icons that you can put on your own web server and use for linking to your FOAF file. Ian lists these on this page but the GIF versions are inlined here (see Ian's page for an SVG version created by Jim Ley):
See FOAFIconIdeas for some other suggestions for FOAF icons.
Morten Frederiksen has also created some icons for linking to the FOAF Explorer
Here are some Christmas images based on Ian's design.