I'm not familiar with this theme or it's designer, but it seems a little sloppy. This, for example, looks bad:
<h1 class="storytitle" id="post-9"><a href="http://nerdette.co.uk/blog/?p=9" rel="bookmark">Coming along</a></h1></div>
There you have what seems to me to be an errant
</div> which appears to be causing your border problem. (Unless that's meant to close the nameless div that appears to be intended to surround each post. In which case, it's doing that but the elements of the post are no longer bound together).
</div> should fix your issue with the border.
The image issue is a little more confusing. The simplest solution is just to make sure that your post is always long enough to flow around any picture you have in a post.
Barring that, you could add a
clear: left CSS element to your
feedback class. That would usually make the element stay below anything that might be to the left of it but above it in the code.
That doesn't quite work with your theme, because the #content element isn't floated. But if you change the margin line which looks like this:
margin:10px 0 0 155px;
to look like this this:
margin:10px 0 0 5px;
You should have your second post well below the first.
Wow that took a while! Hope that fixes your problems.