I believe I have found the location depicted in the painting. Strangely enough, a moth that looks strikingly like the ones carved into the picture frame is embedded in the wall here. I wonder if there's a way to dislodge it and bring it back to the painting.

Guide

1. Look at your right to see stairs leading down to a big gate.

2. Go on the stairs and look at your left to see a moth between the stairs and a fat guy's statue.

3. Use the Lumos spell on the Moth and then walk back to the mirror.

• There are 19 more Moth puzzles in the Hogwarts Castle and the Hogsmeade.