Posts with thumbnail images overlapping

When I create a post with a thumbnail image, unless there is enough text next to the images, the previous post overlaps

like this http://tomcecil.co.uk/wordpress.jpg

I would like the previous post to start after the image, like this http://tomcecil.co.uk/

I’m using the K2 theme and have done this by adding a 150px bottom margin using

.entry-content {
font-size: 1.2em;
line-height: 1.8em;
text-align: justify;
color: #444;
margin-bottom: 150px;
}

But this doesn’t really work as it leaves big spaces between posts which have more text.

Anyone know the answer…this is killing me!

Best

Tom