round image

Hi!
Please check http://www.acupunctuur-shiatsu-veldhoven.nl/wp/artikelen/
Is it possible to make these images round?
When you click on an article, the full article opens. The image there is also round.
How can I do this? Tried this code but does not work:
.body.page-id-21 .slide-image {
border-radius: 200px !important;
}

Martine