Actually you have not set featured image for the pages displayed in slider. Display posts widget fetch image set as the featured image for the post/page.
1. Simple instructions to set featured image can be found on following link,
https://www.wpexplorer.com/featured-images-wordpress/
2. Featured images will also show on page/posts content page (where main content of the page is shown). However, you can easily hide that using following method,
How to Hide featured image from single page/post.
1. Go to WordPress Dashboard > Appearance > Customize.
2. Now go to “Theme Options > General Settings” in left sidebar.
3. Uncheck “Show Thumbnail (Featured Image) on Single Posts” option and publish/update your changes.
Above steps should help you to display slider and single posts properly.
