A couple of issues:

1. You are using CSS selector to float any image that appears in your blog post.  div.blog_body span img {} This approach seems like you will end up floating images that you don't intend to float.

2. You need to using a clearing element or use a clear fix class like dnnClear on the containing element.  In this case when I look at the HTML I see a < span > < img /> </ span >< p > So your choice is to include the dnnClear class on the span or on the p element.  This will ensure that any subsequent content is not affected by the float.