I don't much like the idea of creating a whole web browser instance just to convert an image. I'd recommend you find a library that can render SVG images to bitmap data, then convert that data to a JPEG using .Net's built in image encoding classes.
Render SVG: http://www.codeplex.com/svg
Encode JPEG: http://msdn.microsoft.com/en-us/library/aa970689.aspx
I think that page shows how to encode images using file streams (i.e., the image has to be on the file system before you encode/decode it). However, I'm pretty sure there are overloads for memory streams (so you can do everything in memory without ever touching the file system). I've never had personal experience with the SVG rendering library I posted above... was just the first thing I came across when I did a Google search.