But I don't know how to do it in NoF.
But here's an idea... if you have Photoshop, add a second layer above the image. Fill this with white, then drop the opacity of this layer down to, say, 50%.
This will give the illusion that you want: click here
If you don't have Photoshop I'd be very happy to do this for you - it would take a matter of moments.
It's also far, far easier than fiddling about with text box opacity. If you do this, then the text itself takes on the opacity, too! There's a fix, but it's a bit fiddly.