Want to center an image? Very easy, Bootstrap comes with two classes, .center-block and text-center.
Use the former in the case of your image being a BLOCK element, for example, adding img-responsive class to your img makes the img a block element. You should know this if you know how to navigate in the web console and see applied styles to an element.
Don't want to use a class? No problem, here is the CSS bootstrap uses. You can make a custom class or write a CSS rule for the element to match the Bootstrap class.
// In case you're dealing with a block element apply this to the element itself
.center-block {
margin-left:auto;
margin-right:auto;
display:block;
}
// In case you're dealing with a inline element apply this to the parent
.text-center {
text-align:center
}