+1 - pulling it into the cache and refreshing on a reasonable basis (not sure what that would be) seems like the best approach unless it presents significant problems to do. Mostly for the "doesn't die when the remote site is down" aspect.

Of course, that would create a lag (of, at a maximum, whatever length of time was a 'reasonable basis') between changes to a remote site's user icons and the update appearing on Dreamwidth.