who is that graffiti artist that famously put his own work up in the Tate?

Answer 1

The man's name is Bansky.

He snuck into the museum in disguise and glued the painting to the wall. It was only discovered when the glue came loose and the painting fell to the floor. It is now currently residing in the museum's 'lost property' room.

The painting was of a rural scene with police tape stenciled over it. There was also a card which read:

"Banksy 1975. Crimewatch UK Has Ruined The Countryside For All Of Us. 2003. Oil On Canvas.

"The artist has found an unsigned oil painting and then stencilled police incident tape over the top.

"He argues that ruining the work in this way reflects how our nation has been vandalised by an obsession with crime and paedophilia."

Link 1 below is to that particular news story, link 2 is to Bansky's web site.