There can be issues, after configuration changes, when WordPress isn't able to display the thumbnails of previously uploaded images. You can use the Regenerate Thumbnails plugin to fix this issue.

Install the plugin in WordPress.

Activate the plugin in WordPress.

If you want to regenerate all of the thumbnails for your site:

Under the Tools menu, click on Regen. Thumbnails

Click on the Regenerate All Thumbnails button.

If you want to update individual image thumbnails:

Click on Media from the WordPress navigation bar.

If you are in the Grid view, you will need to click the List view in your Media Library.

Locate the image and hover over it so you can click on the Regenerate Thumbnails link.

The steps above will leave you at a page that shows the success of regenerating the images.

Note: This WordPress plugin is a third-party plugin, please review the developer's page for issues with it's usage.