How to make a circular image in css

Question

This is my problem, although I'm really new to this kind of thing. I've read over a few questions, and while the method for making it circular sounds sensible, the image that is created is only partially round. Is there a way to rectify this? I'm utilizing CSS and HTML.

circular_image {
  float: left;
  margin-left: 125px;
  margin-top: 20px;
  width: 200px;
  height: 200px;
  border-radius: 100%;
  overflow: hidden;
  background-color: blue;
}