The discussion about "built-in" bitmaps seems to have forgotten URNs.
By designating the common bullets/icons as URNs it all becomes a mere
matter of caching strategy.
The URN mechanism provides a protocol for resolving a URN to the nearest
available URL, similar I guess to the domain name service for host names.
By making the local URN name server cooperate with a local HTTP server,
frequently requested URNs will be served directly from the local cache.
The cache management strategy doesn't have to be completely automatic,
and you could designate some objects to be cached using a config file.
For red-hot performance, the first point of access for the URN name service
would be a permanently loaded process on your own machine (avoiding inetd
fork/exec delay), with objects in the local cache being accessed directly
as files.
Dave