Yea thats a cool idea.
and it could work, too!
right click on the image and go to properties
there will be a line that says "Opens With:_____ Change"
click on change.
then browse for Paint.NET. once you find it, click on the checked box i've selected, so that type of file (JPEG, PND, etc.) always gets opened with Paint.NET
hehe just figured out how to take screenshot
i dont know if that is what you wanted, but now if you click on an image, or right click and go to Edit, it opens it in Paint.NET.