Not all templates use the Promoted Block feature. The ones that do, show the first image in a special way that makes it stand out.Read More
In this template (Five), you can display a thumbnail image next to the excerpt. Not all templates do that. You can see which ones do on my Squarespace template comparison chart (also located in my book). Even if your template doesn't do that, you can recreate a very similar layout using a Summary Block. Be sure to scan through all 8 posts in this sample Squarespace blog because each one shows a few different settings/options/content types.