Looking again at your image it appears the 'Preview' button at the top of the Window is on the wrong line, it should be displayed inline with the Image URL text box.
I suspect that maybe your browser settings are causing this, for example if you have the default fount size set larger than normal in your browser it can push some elements to new lines like it is doing in this case.
The only thing could possible try is for the next release making the Image URL text box smaller so that it does not push the 'Preview' button to a new line.
The Rich Text Editor is included with Web Wiz Forums. It is a specially adapted version of our standalone version, which has many modifications to allow it to run better within Web Wiz Forums and also integrate the forum permissions system and security.