I assume, you want to save on client side some sort of formatted text as image. There are ways to do that, but I suggest you do it on server side, and send it to the client as content. First of all, because you have more possibilities in a server os than in silverlight. And the content is also on the server, as you stated.
On server side you can call any tool, even IE - but that's complicated. You can simply spawn a process and call a command line tool that generates the image for you. If this could last long time, you might use some async callback to the client (but there are other options too). I have not found any free cli tools for windows, but commercial ones (http://www.coolutils.com/HTML-PNG-CommandLine
] or http://www.verypdf.com/app/html-to-any/index.html
But you can also make your own tool (or no tool, just generate the image), starting from this great article: A Professional HTML Renderer You Will Use