g1smd - 11:51 am on Mar 14, 2013 (gmt 0)
In general I would use hyphens rather than underscores, especially for page URLs. See also the recent discussion about having the ID number at the beginning rather than the end of the URL.
You don't need to rename the actual image file. URLs are a reference system used on the web. Filenames exist only inside the server. It is your server configuration that finds the right file when a particular URL is requested.