What do I do with the painting?

Hint 1 of 9:

First you have to silence Galipo.

Hint 2 of 9:

Once Galipo is asleep, you can look at the painting.

Hint 3 of 9:

There is one thing strange about the painting.

Hint 4 of 9:

Click the mouse on the firing cannon.

Hint 5 of 9:

The painting will descend, revealing a code box.