create a copy of the configurable.css file as configurableHTMLEditor.css
In the configurableHTMLEditor.css file, remove the background image from the body
on each of the pages that has the HTML Editor instance, find the following code:
$CKEditor_config["contentsCss"] = array("../webassist/themes/". ($WAGLOBAL_Page_Theme) ."/css/configurable.css");
and change it to point to the configurableHTMLEditor.css file instead:
$CKEditor_config["contentsCss"] = array("../webassist/themes/". ($WAGLOBAL_Page_Theme) ."/css/configurableHTMLEditor.css");