image::resize [ -item_id item_id ] [ -revision_id revision_id ] [ -size_name size_name ]
and the CR automatically creates the new sizes and stores them as children of the item_id.
To retrieve them you call:
image::get_size_item_id [ -item_id item_id ] [ -size_name size_name ]
specifying the appropriate image size or use the virtual url handler you described.
R.