This size is in bytes, and defaults to ( 500 * 1024 ) = 500KB.
For now, this image is selected at random, but a subtler method -- e.g. selecting the least-frequently-used image, or the image that whose last-requested date is earliest -- should be developed soon.
number of cache files that will ever be created.
cache can grow to. By default, this is ( 10 * 1024 * 1024 ) bytes, or 10MB.