Floated images display incorrectly in edit window

I have a page with multiple floated images in it. They display ok on the visitor side of the site, but in the edit window they are messed up, like this: http://s464716271.websitehome.co.uk/images/error-screenshot.jpg If I use the default Twenty Twelve theme everything is ok in the edit window: http://s464716271.websitehome.co.uk/images/ok-screenshot.jpg I have to switch back to Twenty Twelve to edit the page, which is obviously no good for the end user. I'm using version 0.1.5 and a child theme. Even if I use the parent Catch Everest theme on its own the problem still happens. Anybody know how to fix this?