Drop shadow for post images?

I am working on a new theme and trying to add drop shadows to my post images. The drop shadow CSS I have looks like this:

.img-shadow {
  float:left;
  background: url(./images/shadow.gif) no-repeat bottom right;
  }

.img-shadow img {
  display: block;
  position: relative;
  background-color: #fff;
  border: 1px solid #ccc;
  margin: -6px 6px 6px -6px;
  padding: 4px;
  }

In order to get this to work, I need my images inside a div tag. Or at least I think I do. I am kind of new to this. How do I set up my theme so that all of my post images are automatically enclosed in a div tag? I have tried assigning a drop shadow to the image class, but that doesn’t give me the look I want – I want a photo edge look, with a border (as shown in the above CSS).

I hope this makes sense.