How to interpret PSNR value?
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.