You don't want to pollute the preferences with something that's only used during editing sessions. If you make the purge a user-initiated action, then it won't be an annoyance to prompt for which unused images should be kept. One possibility is to hook into the ValidityChecker step (the "Some problems were found with this module" dialog that pops up during a save). If one of the problems is "Some images do not appear to be used" then the user can hit a button to purge them.
rk [Non-text portions of this message have been removed]
