From the screen shots that you provided I can see that you are using old version of RadEditor for MOSS. You can find what exactly is the version by checking the following folder Program Files\Common Files\Microsoft Shared\web server extensions\wpresources\RadEditorSharePoint\5.x.x.x__1f131a624888eeed\Resources (5.x.x.x is the version). In its previous versions RadEditor had some issues with rendering in quirks mode (since it is XHTML based editor). Unfortunately the SharePoint sites are using this type of rendering. What I suggest you to do is upgrade to the latest version of RadEditor for MOSS, which now is 5.4.0.
You can find information on upgrading to a newer version of the control in the following help article http://www.telerik.com/help/aspnet-ajax/upgrade_to_a_newer_version_of_radeditor_for_moss.html
One other solution is to try using some other skin of the control. In order to do so you need to modify the respective ConfigFile.xml or ListConfigFile.xml (for forms such as list, wiki, blog ...), which are located in the mentioned above folder.
For example if you want to set the default skin you need to add the following code.
You can find information on the skins in the following help article http://www.telerik.com/help/aspnet-ajax/skinbasics.html
I hope this helps.
the Telerik team