Not really sure if my comment should be part of this change, but there was a question the other day on StackOverflow about changing the filename (for ImagesPipeline) based not only on URL, but also based on some info from the originating item. One of my ideas was to additionally pass the request or response to `image_key()`. It's helpful in case you add `meta` to `Request` in `get_media_requests()`
My answer to http://stackoverflow.com/questions/18081997/scrapy-customize-image-pipeline-with-renaming-defualt-image-name/18083143 suggests something else though