Yes, indeed there is a problem in the current version that breaks the image rendering when you set the Visible property of HtmlTextBox item to false. The issue has already been addressed and would be part
of the SP1 expected by the end of this week (next week at latest).
For the time being, you can set a minimal height for the HtmlTextBox,
so that it would grow whenever there is content. Then when you want to
hide it, set its Value as empty string and this way it would not grow
i.e. would not occupy any space, just like it was set to visible=false.
Please excuse us for the temporary inconvenience.
the Telerik team