No plugins are required - everything you need is available in paint.net itself, as HyRez said.
Open the image you want to enhance.
Add a new layer above the original image.
Activate the Ellipse select tool (other shapes will also work - e.g. use a vertical rectangle for the side highlights)
Drag out a flattened ellipse where you want the highlight. It needs to be quite thin (reference the top highlight in you example image)
Press Shift + Backspace to fill the ellipse with white.
Apply Blurs > Gaussian Blur. Use the Radius value to soften the edges of the highlight.
Press F4 to open the layer properties dialog and fine tune the effect by lowering the Opacity value.