CSS for images

I am trying to add an image class to the style sheet in my child theme.
As a test, I created this:

img.nature {
margin: 20px;
padding: 20px;
border:solid #003300 4px;
background: #006600;

it doesn’t work if i put it in the Child style sheet, but if I put it in the main Sydney Pro style sheet, it does work. Ideas as to why that is? Thanks!

Disregard (or delete)
Error on my part it appears