Hi briankitt,
Please try this way
1. Create a css file name "example.css" under the root of your site
below is the code in example.css
img
{
border: solid 1px blue;
}
2. Set property EditorWysiwygModeCss to "~/example.css";
<CE:Editor ID="Editor1" runat="server" EditorWysiwygModeCss="~/example.css">
</CE:Editor>
Regards,
Ken