How do I make images appear in the beside blog post summaries?

It’s hard to tell without seeing the theme files but I assume the one image is displaying because it is set as the “Post Thumbnail” or “Featured Image” on that particular post and that all other posts aren’t using that feature.

The easy solution in that case would be to make sure you set any images you use in the post as the “Featured Image” as well.

If that isn’t the case then you can modify your themes function file to grab the first image of any post and then modify your index.php file to include that image if it exists.

Here is a good tutorial on that:
http://www.wprecipes.com/how-to-get-the-first-image-from-the-post-and-display-it