The reason you're not getting any hits is because the default behavior in Volley for disk caching is dependent on the HTTP headers of the element you're requesting (in your case, an image).
Check the volley logs and see if you get the "cache-hit-expired" message - that means that the image was cached but it's TTL is expired as far as the default disk cache is concerned.
If you want the default settings to work, the images must have a Cache-Control header like max-age=??? where the question marks indicate enough seconds from the time it was downloaded.
If you want to change the default behavior, I'm not sure, but I think you have to edit the code a bit.
Look at the CacheDispatcher class in the Volley source.
Hope that helps.