Check the actual source code of the page, not just the developer tools. Yii does generate the tag correctly by default, but the developer tools may alter it.
EDIT:
As an aside, the forward slash is not required in HTML 5 (although it is accepted) as HTML 5 isn’t a form of XML.
Thank you, Keith! Of course, you’re right! I was so devoted to using dev-tools, that I hasn’t been using Ctrl+U for ages! :]
I didn’t knew, that such things happens. What could be the purpose of changing real source code in dev tools? I can’t find any…
I got confused by Netbeans 7.4 marking this as an error (of course, when I wrote <img> tag directly, not when I used CHtml:tag). Didn’t know, that HTML5 doesn’t require ending single-tags. Good to know.