The evaluation of the image data takes place via URLs from your data source. You can, for example, add a link to the employee's profile picture (available online) and then display this picture directly in the detail view of your list.
In the example source file, column R contains the web links to the respective image files of the employee. During app creation, this column is defined as an "image" so that the image is displayed directly and not just a link to the file is visible.
Where can I host my pictures?
For example, you can host your pictures with Dropbox and share them with people outside Dropbox. Create a shared link so the app can access your file via the link. To open the file, no Dropbox account is needed. ("Shared links: Give users read-only access to your files")
The created link looks like this, but has to be adjusted:
You now have to force the file to be downloaded directly instead of just being displayed. You can do this by using
dl=1 as the query parameter in your URL. Example: