The following reasons could be taken in consideration if the images have disappeared from the website:

Outdated Elements:

If you notice that the images have disappeared, the first thing to check should be that everything is up to date: WordPress, plugins and the active theme.

There are cases in which the plugins were updated, but not the platform or theme and vice versa. In some cases an old plugin version along with an up-to-date theme/platform can cause faulty functionality.

After you make sure of that, check the website from Chrome -> Incognito mode or Safari -> Private Window to be sure that you are not viewing a cached version of your website.

Plugin Conflict:

One of the most encountered situations is when a plugin is conflicting and it blocks the images to be visible on front end (what visitors see).

To check this, deactivate all the plugins with no exception and navigate to the website from a Private Window or Incognito mode in Chrome.
If the images are present, it means a plugin is conflicting and you can activate each one, refreshing the website after each activation to see which one is the culprit. 

You can find an alternative plugin at www.wordpress.org/plugins

Server Related Issues:

There are situations in which the images are missing due to the hosting services and user permissions or space on the server where the website is hosted.

In this situation, getting in touch with the hosting company and telling them to check should fix the issue.

Deleted Images:

Another situation is when the images were simply deleted from the server, manually to create some space, without knowing that can affect things.

Example:

You add an image to a page either using The Divi Builder, or the Default WordPress Editor.

When that image is added, it's being uploaded via the Media Library.

In that moment WordPress automatically creates a folder on your server where it stores all the images uploaded through the Media Library.

Example: \wp-content\uploads\2018\07\myimage.jpg

That means in July of 2018 the image named myimage was uploaded. So what the visitors are seeing now when are viewing that page, is the image served from that path.

If in the meantime that image is deleted from the server for any reason (manually or by an optimization plugin), the path still remains active in front end when you access the page, but with the image missing now from the server, there will be nothing to show for the visitors. 

You can check that by inspecting the page where the image is missing and check the path: 

In this case simply re-uploading the image will create a new path from which it will be served.