Custom CSS Class To Remove Image Border

Hi there!

I'm applying a rounded black border to my images:

#page img {
border: 1px solid #000000;
border-radius: 10px;
}

This works, but now I need to remove that border from some images (diagrams). I thought I could create a special class called "no-border-image" and add it to the images.

I tried adding this to my child theme:

.no-border-image {
border: 1px solid #ffffff;
border-radius: 10px;
}

It doesn't work. Any idea what would, please?

Thanks,
Oliver