Hello,
I can only give you a partial answer…
To add an image to readmore-button in general you can add this style-rule to template.css (around line 45):
a.readon span{padding-left:12px; background:url(../images/icon-arrow.gif)no-repeat left}
As an example; this will give an arrow before all readmore-links.
But next step: how to select only the articles which have access level registerd, I don’t know.
One cannot give an individual article a class or id, only the link to this article.
So one can give a Page Class Suffix to a menu-item. But when you have menu-items that link to sections or categories, you cannot select just the articles from a category which have access level registered.
Hope this helps anyway.
Micrantha