The Salal theme includes a nice widget for you that displays the most recent posts with thumbnails. The thumbnails are automatically generated from the featured image you upload for the post(s). However, if you don’t have one, a placeholder image takes the place of one.
Go to Appearance >> Widgets
Drag the Salal Recent Posts widget into the sidebar position you want to show it in, and then use the settings to title it and how many posts to show.