creating a child theme, css not working

Hi guys,
I’m totally new to wordpress, html etc.

I am in the process of creating a child theme.
I have linked the style sheet to the parent style.
Now I am wishing change some of the styles- remove a drop shadow and remove the rounded edges from an image to be specific.

I have copied the part of the coding i wish to change from the parent css and pasted it into the child theme css, then removed the drop shadow and rounded edge values. Yet the design on the web page remains the same.
Below is the coding:

/*

Theme Name: Twenty Twelve Child
Theme URI: public_html/wp/wp-content/themes/twentytwelve
Description: Twenty Twelve child theme with a twist or two
Author: shaun baldwin
Version: 0.0.1
Template: twentytwelve

*/

@import url('../twentytwelve/style.css');

.entry-content img, .comment-content img, .widget img, img.header-image, .author-avatar img, img.wp-post-image {
	border-radius: 0;
	box-shadow: none;
}

I’m using text wrangler to write the code and uploading the code using cpanel.

Any help would be hugely appreciated. As mentioned earlier i’m new to all this so my understanding of wordpress in minimal.

Thanks in advance,
Shaun