Have you modified any theme files?
Please check after disabling all plugins and creating a new post with featured image.
The image source shows something like below – are you using Jetpack Photon service?
data-orig-file=”https://i0.wp.com/www.myrandomtips.com/wp-content/uploads/2017/10/Get-KeePass-Working-on-SharePoint-1174×665.jpg?fit=1174%2C665&ssl=1″
