How to add a div only if image exists in post

Hello,

I am trying to add a design element onto the top of every image that is posted on my blog. This wouldn’t be a problem, I tried just adding some extra padding to the top of each image and then adding the design element as a repeating background image. It works but adding the extra padding ruins the rounded corners that are applied with CSS to the image.

Here is my CSS:

.entry img {
        -moz-border-radius: 18px 18px 18px 18px;
        -webkit-border-radius: 18px 18px 18px 18px;
        -khtml-border-radius: 18px 18px 18px 18px;
        -o-border-radius: 18px 18px 18px 18px;
        border-radius: 18px 18px 18px 18px;
        background-image:url('images/slantedLine.png');
	background-repeat:repeat-x;
	padding-top:20px;
}

Here is a picture showing the rounded corners getting screwed up by padding:
https://i.imgur.com/acrdI.png

I’m pretty sure it isn’t solvable with just css, or at least I can’t figure it out. I think I need to write some php that checks if there are any images in the post and then if there are images in the post add a div element to the top of each of those images. After trying for 2 hours I have realized I just simply don’t know how to do this.

Any help at all pointing me in the right direction would be much appreciated.