In my recent seo research I now realize Google recognizes hyphens as dividers in image names, urls, and body content and ignores underscores. The underscore is not considered a divider (separator) by Google. So use hyphens not underscores in order for the search engines to recognize your keywords. Also keep your urls under 30 characters as it may be problematic for search engines to crawl and index them.

For more info on dashes vs underscores, check out Matt Cutts blog at http://www.mattcutts.com/blog/dashes-vs-underscores/