Depends on the layout of the image and the surrounding container. Either set text-align: center on the surrounding container, or margin-left : auto, margin-right: auto on the element you want to center itself.
In any case, the surrounding element should have a width (100% probably) otherwise there might be nothing to center to
Just came across this problem and found entering the class ‘center-block’ works perfectly!
Read the docs here: https://getbootstrap.com/docs/3.3/css/#images (Use CTRL + F to find ‘center-block’ quickly)