I've done a fair bit of CSS, and I don't remember ever having used or even seen clip. Misunderstood CSS Clip suggests I'm not the only one: "The CSS clip property has to be one of the least used properties in CSS. This is probably because no one really knows when to use it, it doesn't appear to be supported in Internet Explorer, and some people use it incorrectly."
So don't do it with clip. That lets you get rid of position: absolute, which as you recognized isn't what you want. If I understand what you're trying to do, just set width: and height: values for the images, plus some padding, maybe like so:
.Thumbnails {
width: 100px;
height: 100px;
padding-bottom: 10px;
padding-right: 10px;
}
(Eric Meyer's "Cascading Style Sheets: The Definitive Guide" says "The long and convoluted history of clip means that, in current browsers, it acts in inconsistent ways and cannot be relied upon in any cross-browser environment.")