This post at the Google News blog has some answers that could be helpful as you think about optimiziation strategies.
Having an image next to your article improves your ranking MYTH
While having a good image with your article does improve your chance to get your picture shown, it has no impact on the ranking of the article itself. There are some tips in our help center designed to help us include more of your images in Google News. We encourage you to check those out if you have had problems getting images included in the past.
UPDATE: I have elevated this comment from blogger Danny Sanchez, with some amplification:
Having an image may not help the story's ranking BUT it does improve its chances of being the featured thumbnail for a story. That means it will display next to the lead headline and in the regular Google Search results if it triggers a news result.