33. The computer-readable storage medium of claim 32 , wherein the method further comprises: computing a statistical profile for the ages of resources returned from a given subcache in response to a resource query or filtering query using the statistical profile to compute a purgeable age; and updating the purgeable age with the computed purgeable age.
32. The computer-readable storage medium of claim 31 , wherein each subcache includes a maximum age and purgeable age limit, wherein the method further comprises: removing an identifier for an image from the cache if the identifier for the image is older than the maximum age limit, wherein removing the identifier for the image involves removing the underlying image from the location in memory or on disk; marking the identifier for the image as purgeable if the identifier for the image is older than the purgeable age; and wherein when an identifier for an image is marked as purgeable, the method further comprises removing the associated image from the location in memory if the location in memory is required for another purpose.