There is another interface called R-(D)Com, which might be helpful. See this question for someone using it with C#. I have no idea whether it will work for your use case, though.
If there isn't an existing interface that can do what you want, this probably isn't going to be very easy to do. You could run R as an external command, which would probably be easy, but slow and inelegant. Or you could work on the R.NET source code to add the required functionality, which might be a big job.
Depending on your needs I would be more inclined to take one of the following approaches:
- Port your image processing algorithm to .NET and remove R from the equation altogether.
- If it is a desktop program, one option is making the entire program in R (R has Tk built in, which allows GUIs to be created easily.).
- Instead of .NET, use something else that has a better interface to R.