explain these linesThat's my last duchess painted on the wall, Looking as if she were alive. I call That piece a wonder, now: Frà Pandolf's hands...

Here the Duke is showing the painting of the Duchess to
the messenger and is explaining that it has been completed very successfully by 'Fra
Pandolf'. The Duke normally looks at the painting alone and in private as it is hidden
behind a curtain but he is allowing the messenger to take a look as he is so proud of
it: 'The depth and passion of its earnest glance.'

The
irony is that the painting looks alive when in fact we learn later in the poem that the
Duke has ordered her to be killed; he prefers her as a piece of art that he can
control.  As a painting he can control who looks at her which was not possible in life.
 The Duke has tremendous power which is why people do not ask questions of
him.