I do not see any overlap between the thumbnails and the post summaries on Firefox, Chrome, or IE 9.
As Larry mentioned, if you want all of your thumbnails to stretch across the width of the content area, you need to use a featured image that is at least the same width of your main content area (620px).
Or, you can also add this to the end of the various.css virtual CSS file:
Code:
.thumb-shadow {
float: left;
}
Then the borders on all of your smaller thumbnails will at least fit tightly around the image instead of stretching across the entire width.