Adding Custom Image Styles to the Ribbon

Question

I use the following Editor Control, with the PrefixStyleSheet "el-" for my custom styles:

<PublishingWebControls:RichHtmlField FieldName="f55c4d88-1f2e-4ad9-aaa8-819af4ee7ee8" HasInitialFocus="True" MinimumEditHeight="400px" PrefixStyleSheet="el-" runat="server" />

And all my custom Markup Styles are working, like this:

H1.el-Element-h1 {
  -ms-name: "Heading 1";
  border-bottom: 1px solid #AF0917;
  color: #AF0917;
  font-size: 22px;
  font-weight: normal;
  padding-bottom: 3px;
}
...

but the image styles don't work:

.el-rteImage-0 {
  -ms-name:"No Border";
}
.el-rteImage-1 {
  -ms-name:"Border";  
  border: 1px solid #ccc;
  padding: 2px;  
}

What did i wrong?