>>Not interested in the image,

backgroundimage.css:

body
{
 background-image:url("/images/splash.jpg") ;
 background-repeat:no-repeat;
 background-position: center;
 background-color: red
} 

We will add a new property in the next minor version which allows you set editor background color.

>>And while I'm at it, where can I set the default height and width?

Do you want to change the editor width and height? You need to change the Cute Editor provider for DNN3.

>>Can this stuff be defined in the skin.css file at all?

What do you want to define in the skin.css? The  editor background color? I don't think that's a good idea.

>>What file do I need to edit to define the cssclass for the tags?

Editor provider for DNN. The file name is CEHtmlEditorProvider.vb.

            Dim s1, s2, s3, s4 As String

            s1 = Common.Globals.HostPath & "default.css" 'default css
            s2 = PortalSettings.ActiveTab.SkinPath & "skin.css" 'skin css
            s3 = (PortalSettings.ActiveTab.SkinSrc).Substring(0, PortalSettings.ActiveTab.SkinSrc.LastIndexOf("/")) & "/skin.css"
            s4 = PortalSettings.HomeDirectory & "portal.css" 'portal stylesheet

            'initialize the control
            If cntlCE.EditorWysiwygModeCss = "" Then
                cntlCE.EditorWysiwygModeCss = s1 & "," & s2 & "," & s3 & ", " & s4
            End If