This is called The Druids Bringing in the Mistletoe by George Henry (1890)

It’s in the Kelvingrove Art Gallery and Museum, Glasgow,
a fine museum with an important Monticelli, as well,
which, if Sir Timothy has anything to say about it,
will never be put on display.

The Monticelli in Glasgow is a second version
of the Monticelli that hangs over the wizard’s desk.  
Below is a detail from the Glasgow version.
Wizzo’s Monticelli is beneath it.