Trouble with image caption

Thanks a lot for this wonderful theme.
I’ve trouble with image captions.
When I insert in an article an image, via WP inject, it is correctly displayed as in this screenshot: http://postimg.org/image/ithcofwxl/
Unfortunately, when I preview the article (or publish it), caption is displayed in a different way, as in this screenshot: http://postimg.org/image/4objmmnw9/.

How can I fix it?
thanks in advance
Ferruccio
P.S: I’m using FF 40 on Windows7