Is there a way to explicitly clear the cache used by the UriImageSource?

Our app has a requirement to clear cached images only when the user logs out, but I don't see a way to do it without "cheating" and explicitly accessing the "ImageLoaderCache" folder in IsolatedStorage.

I imagine we could set the CacheValidity to a TimeSpan of 0 (or equivalent) for every image when the user logs out, but then we'd have to keep track of every image pulled from the web.