How to interpret PSNR value? Say, I have original image, noised and denoised. By PSNR value I want to find out how good does my denoising algorithm works.

For which pair of the images should I calculate it? I've tried it for original+noised, noised+denoised and original+denoised and got PSNR = 28, 29 and 31 respectively.

What value shall be appropriate?

I'm calculating PNSR as here.
With original image $o$ fixed, the higher, the better.
If PSRN($o,d$) is bigger than PSRN($o,n$), you can claim an improvement in the decibel measure. It is difficult to interpret PSRN($d,n$), sinceyou compare two more-or-less noisy data.

Is PSRN a good measure of subjective image quality remains an open question.