Posts using the “Gallery” choice under the Format menu do not show images, unless you set a featured image. You have to click on the actual post to show the image. Also, some frames do not show post titles. Customizability is limited.
This is the Gallery format. To select this frame, choose “Gallery” on the right, under the Format menu. Images do not show up in posts with this format unless you open the actual post, or set a featured image. To open the post, click on the post title, “Gallery.”