Images appear with source code

Since the update to 3.0.4, every time I try to upload (or call a library image) to a post or page, all the source code related to the image shows up as well.

I’ve tried everything I can think of, but nothing fixes it. I can edit the code in HTML, but the moment I click “Update” or “Publish”, the source code becomes visible (plus the image itself), usually without the first <img tag and ending with > instead of the > closing tag.

Is this a bug in the code, a plug-in conflict or what?