This Google support page says there are multiple reasons this might happen:
- Use standard image formats, like JPEG, and standard filename extensions, like .jpg or .jpeg.
- Make sure that your images are fairly large in size, at least 60 pixels by 60 pixels.
- Use images that have reasonable aspect ratios.
- Ensure that your images are inline.
- Ensure that your clickable images link to a URL with a .jpg or .jpeg extension.
- Place your images near their respective article titles.
- Label your images with well-written captions.
- Ensure that a robots.txt file isn't blocking our access to your images.
- Make sure your images are on a domain included in Google News. If your images are hosted on a separate domain from your site's main domain it's very unlikely we'll be able to crawl them.
And to top it off:
Please keep in mind that in order to show as many different sources to our users as possible, we sometimes pair relevant images with articles from different sources.
So even if you do everything right, the pictures may still not show. But if you follow these guidelines this will probably barely happen.