PictureBox and css class for rounded image

Hey

When I use a picturebox with an image in it, how can I achieve it, to add css to the image, so that it is rounded?

I added a StyleSheet component and added the following to the CssClass:

img {border-radius: 50%;}

But nothing happens…

Could you please give me a hint, how to achieve this.

THX a lot – Have a nice weekend