Cache_Graphics and expiration

Hi everyone! I've just started doing work with PEAR::Cache, and it's fantastic! I really like it. My problem is this- I am using the Cache_Graphics class to cache the output of some images that are being created by GD, and is being assigned a specific ID from an MD5 hash. There are two things I want to do: I want to set it so that these cached image files expire after a certain period of time (say, 24 hours), and also I want to be able to expire them manually. Say for example, I have a page that generates a graphic displaying relationships between users. I might want this to be refreshed every day (the automatic expiration), and also whenever the relationship data changes, I want it to know that now it needs to re-create the image instead of using the cache. From what I understand, if I use output buffering and the Cache::save() function, this will allow me to set an expiration time, but I'm using the Cache_Graphics::getImageLink() function, because it's important for me to not have the additional page hit (which is what would happen if I had the <img> tag go to another page to do the output buffering). Any thoughts? Thanks a lot, Jason